Using SNOMED CT in HL7 Version 3; Implementation Guide, Release 1.5

Each SNOMED CT concept is defined by relationships to one or more other concepts. The following example illustrates the type of logical definitions that are distributed as part of SNOMED CT.

[ 71620000 | fracture of femur ] is fully defined as... 
   116680003 | is a | = 46866001 | fracture of lower limb | ,
   116680003 | is a | = 7523003 | injury of thigh | ,
     116676008 | associated morphology | = 72704001 | fracture | ,
     363698007 | finding site | = 71341001 | bone structure of femur |

NOTE: This example and many of the other illustrations in this document are expressed using the SNOMED CT compositional grammar. Where relevant this document also uses proposed extensions to this grammar to represent constraints on use of SNOMED CT concepts and expressions. The extended grammar is explained in SNOMED CT Compositional Grammar - extended (§ B.3 ), together with references to the SNOMED CT source material.

When a SNOMED CT concept is used to record an instance of information, it can be refined in accordance with the SNOMED CT Concept Model to represent more precise meanings.

• For example, it might be necessary to record a "compression fracture of the neck of the femur".
  • SNOMED CT does not contain a concept identifier for this specific type of fracture at this precise location. However, the post-coordination rules allow refinement of the "finding site" and "associated morphology" attributes in the definition of the concept "fracture of femur" (see above example).
  • Therefore the required information can be recorded by refining the concept "fracture of femur" with the site "neck of femur" and the morphology "compression fracture".

The result of a refinement is referred to as a post-coordinated expression. A post-coordinated expression conforms to an abstract logical model specified the "SNOMED CT Guide to Abstract Logical Models and Representational Forms" (see SNOMED CT Reference materials (§ B.2 )). The same guide also specifies a compositional grammar for representing these expressions in a way that is both human-readable and computer-processable (see also SNOMED CT Compositional Grammar - extended (§ B.3 )). The example below uses this grammar to represent a post-coordinated expression for "compression fracture of neck of femur".

71620000|fracture of femur|:
   116676008|associated morphology|=21947006|compression fracture|
   ,363698007|finding site|=29627003|structure of neck of femur|

The composition grammar illustrated above is only one of the possible representational forms for a post-coordinated expression. These expressions can also be accommodated within the HL7 Concept Descriptor (CD) data type which may be applied to various coded attributes in HL7 specification. For example, the SNOMED CT expression indicating a "compression fracture of neck of femur" can be represented as shown in the following example:

<code code="71620000|fracture of femur|:116676008|associated morphology|=21947006|compression fracture|,363698007|finding site|=29627003|structure of neck of femur|" 
      codeSystem="2.16.840.1.113883.6.96"/>

SNOMED CT "clinical finding" and "procedure" concepts have assumed (default) contexts which apply if they are used in a record without an explicit context.

• The default context for a [ <<404684003 | clinical finding ] is that the finding is asserted to be present in the person who is the subject of the record at the current time (or at a specified time).
  • E.g. When the concept [ 233604007 | pneumonia ] is used in a clinical record it is assumed to mean that pneumonia was found to be present in the subject of the record either at an explicitly stated effective time or at the current time when the statement was made.
• The default context for a "procedure" is that the procedure is asserted to have been done to the person who is the subject of the record, at the current time (or at a specified time).
  • E.g. When the concept [ 80146002 | appendectomy ] is used in a clinical record it is assumed to mean that an appendectomy was done on the subject of the record either at an explicitly stated effective time or at the current time when the statement was made.

The default context for a [ <<404684003 | clinical finding ] can be overridden by an explicit representation of context. Alternative contexts include:

• Finding contexts such as: present, absent, unknown, goal, risk, etc.
• Subject relationship contexts such as: family member, mother, father, sibling, contact, etc.
• Temporal contexts such as: past, current, recent, etc.

The default context for a [ <<71388002 | procedure ] can be overridden by an explicit representation of context. Alternative contexts include:

• Procedure contexts such as: requested, planned, in progress, done, not done, not to be done, etc.
• Subject relationship contexts: as above for findings
• Temporal contexts: as above for findings.

Explicit context may be represented either in a pre-coordinated form using a concept that is a subtypes of [ <<243796009 | situation with explicit context ] or by using a post-coordinated expression.

• The following concepts provide examples of pre-coordinated concepts that include explicit context:
  • [ 297243001 | family history of pernicious anemia ]
  • [ 160274005 | no family history diabetes ]
  • [ 399211009 | past history of myocardial infarction ]
  • [ 168748001 | mammography requested ]
  • [ 165017002 | lung function testing not done ]
• The following expressions illustrate ways in which in post-coordination can be applied to represent explicit context:
  • [ 281666001 | family history of disorder | : 246090004 | associated finding | = 84027009 | pernicious anemia ]
  • [ 417662000 | past history of clinical finding | :246090004 | associated finding | = 22298006 | myocardial infarction ]
  • [ 413350009 | finding with explicit context | : 246090004 | associated finding | = 282144007 | able to walk | , 408729009 | finding context | = 410518001 | goal ]

SNOMED CT expressions can be compared by applying "normal form" transformations that make use of logical concept definitions. These transformations generate the same normal form when applied to two expressions that logically have the same meaning.

• When the transformation rules are applied to either of the following two expressions:
  • [ 297243001 | family history of pernicious anemia ]
  • [ 281666001 | family history of disorder | : 246090004 | associated finding | = 84027009 | pernicious anemia ]
  the following normal form is generated
  • 243796009 | situation with explicit context | :
      {246090004 | associated finding | = 84027009 | pernicious anemia | :
        ,408729009 | finding context | = 410515003 | known present | ,
        408731000 | temporal context | = 410512000 | current or specified | ,
        408732007 | subject relationship context | = 303071001 | person in the family | }

The expressivity of SNOMED CT is one of its strengths. However this also leads to cases where overlaps may occur with semantics that may also be represented by an information model such as the HL7 RIM. For example:

• a single SNOMED CT coded expression can represent a meaning that the HL7 RIM could also represent using a combination of several coded attributes or related classes;
• HL7 RIM semantics may modify the default assumptions about the meaning of a SNOMED expression;
• HL7 RIM semantics may contradict the meaning expressed by a SNOMED CT expression.

There is a requirement for clear rules and guidance on these overlaps to minimize the risk that alternative representational forms, may lead to duplication, ambiguity and erroneous interpretation.

The Act.classCode has the effect of specializing the Act class and, as a result, constrains the vocabulary domains that apply to other coded attributes of that class. If a SNOMED CT expression is used to encode the values of one or more of these other attributes, the meaning of the expression must be appropriate to the constrained vocabulary domain.

The vocabulary domain constraints applicable to specific SNOMED CT encoded attributes of different HL7 classes are specified in SNOMED CT vocabulary domain constraints (§ 5 ).

The rationale for the vocabulary domain constraints applicable to particular HL7 classes are discussed in SNOMED CT vocabulary domain constraints (§ 5 ). This is supplemented by Detailed aspects of issues with a vocabulary specification formalism (§ E ), which discusses different ways in which constraints may need to be expressed to take account of the SNOMED CT terminology model.

A SNOMED CT expression can be used in the Act.code to represent the nature of the action (e.g. using concepts from the hierarchy). In cases where an observation results in a non-numeric result this can also be represented using a SNOMED CT expression. Actions involving measurement or a property or observation of a specified quality can readily be represented unambiguously using this pair of attributes.

Some kinds of observation are typically expressed in a way that either does not specify the action but merely asserts a result (or finding). In these cases the asserted result is fully specified and does not require a detailed indication of the action taken (e.g. "abdomen tender", "past history of renal colic", etc). SNOMED CT supports representation of these assertions in a single expression using concepts from the [ <<404684003 | clinical finding ] and [ 413350009 | finding with explicit context ] hierarchies. Several different ways of representing the same information thus exist using different combinations of the Act.code and Observation.value. Unconstrained use of the alternatives presents a major challenge for computation of semantic equivalence and that for safe interpretation of observations origination from different applications and users.

The following rules are intended to support validation and consistent interpretation of particular combinations of the Observation.code and Observation.value attributes where SNOMED CT is used in either or both of these attributes.

1. In a constrained information model or template that permits or requires the use of SNOMED CT to represent the nature of an Act class clone:
   • the Act.code attribute SHOULD permit the use of the Concept Descriptor (CD) data type.
     • This is required to allow inclusion of post-coordinated expressions using qualifiers where these are appropriate.
   • the Act.code attribute MAY be constrained to a data type that prohibits qualifiers, only if there is known to be no requirement for representation of meanings that might require the use of post-coordinated expressions.
2. In a constrained information model or template that permits or requires the use of SNOMED CT to represent the result of an Observation class clone:
   • the Vocabulary Domain (or value set) specified for the Observation.code attribute SHALL permit the use of the HL7 code value "ASSERTION".
   • the Observation.value attribute SHOULD permit the use of the Concept Descriptor (CD) data type.
     • This is required to allow inclusion of post-coordinated expressions where these are appropriate.
   • the Observation.value attribute MAY be constrained to a data type that prohibits qualifiers, only if there is known to be no requirement for representation of meanings that might require the use of post-coordinated expressions.
3. In an Act class instance where the Act.code attribute is a SNOMED CT expression
   • the expression SHOULD represent a type of [ 363787002 | observable entity ], [ <<129125009 | procedure with explicit context ] or [ <<272379006 | event ]
     • Note: [ <<272379006 | event ] concepts are included here as they may be regarded as have context similar to procedures (for example to be continuing or completed). However, this is an open issue on the SNOMED CT Concept Model, since at present no relationship between [ <<272379006 | event ] concepts and context attributes is currently specified.
4. In an Observation class instance where the Observation.code is the HL7 code "ASSERTION" and the Observation.value is represented by a SNOMED CT expression:
   • the concept represented SHALL be a type of [ <<404684003 | clinical finding ], [ <<413350009 | finding with explicit context ] or [ <<272379006 | event ].
     • Note: [ <<272379006 | event ] concepts are included here as they may in future require representation as assertions that an event occurred or did not occur. However, this is an open issue on the SNOMED CT Concept Model, since at present no relationship between [ <<272379006 | event ] concepts and context attributes is currently specified.
5. In an Observation class instance where the Act.code attribute is a SNOMED CT expression representing a [ <<404684003 | clinical finding ] or [ <<413350009 | finding with explicit context ], if the Observation.value is omitted, the Observation SHALL be interpreted as semantically equivalent to the same SNOMED CT expression in the Observation.value attribute with the Act.code "ASSERTION" (see point 4 above).
   • This deprecated form of representation is permitted to support backward compatibility with existing implementations.
   • For example:
     • <observation><code code=[ 195967001 | asthma ]/>...</observation>
     • is treated as equivalent to
     • <observation><code code="ASSERTION"/><value code=[ 195967001 | asthma ]/>...</observation>
6. An Observation class instance in which the Act.code is a SNOMED CT expression representing a [ <<404684003 | clinical finding ] or [ <<413350009 | finding with explicit context ] SHALL NOT contain an Observation.value attribute.
   • If a value attribute is applied to a [ <<404684003 | clinical finding ] there are multiple possible interpretations of what that value means. For example, the possible meanings of a value applied to a clinical finding such as [ 195967001 | asthma ], [ 195114002 | acute left ventricular failure ] or [ 254838004 | carcinoma of breast ] might include severity, stage, duration, certainty, presence or absence. Thus in this context, the meaning of the value is ambiguous and open to misinterpretation. Further more, such misinterpretation might fundamentally alter the intended meaning. The SNOMED CT Concept Model and HL7 attributes provide ways to explicitly state these nuances of meaning. Therefore use of the non-specific value attribute is not appropriate.
   • In contrast, a value applied to an [ <<363787002 | observable entity ] clearly represents the observed quantitative or qualitative value of the specified entity. Similarly a value applied to a [ <<386053000 | evaluation procedure ] clearly represents the quantitative or qualitative result of that measurement.
7. An Observation class instance in which the Observation.value is a SNOMED CT expression representing a [ <<404684003 | clinical finding ] or a [ <<413350009 | finding with explicit context ] MAY contain an Act.code other than "ASSERTION" provided that the interpretation of the Act.code together with the Observation.value does not yield a meaning that is substantially different from the meaning implied if the Act.code was "ASSERTION". Observations of this type SHOULD be interpreted as having a meaning that is equivalent to the meaning of the same
   • This deprecated form of representation is permitted to support backward compatibility with existing implementations.
   • For example:
     • <observation><code code=[Abdominal examination]/><value code=[Abdomen tender]/>...</observation>
     • does not differ significantly from the asserted observation ...
     • <observation><code code="ASSERTION"/><value code=[Abdomen tender]/>...</observation>
   • In addition, the same Observation class instance can separately be interpreted to determine that an "abdominal examination" was carried out.
     • In the preferred representation this information would be expressed in a separate Act class instance because it relates to a general examination procedure which may have resulted in several distinct assertions.
8. An Observation class instance in which the Observation.value is a SNOMED CT expression representing a [ <<404684003 | clinical finding ] or a [ <<413350009 | finding with explicit context ] SHALL NOT contain an Act.code which when interpreted with the Observation.value yields a meaning that is substantially different from the meaning implied if the Act.code was "ASSERTION".
   • For example, an Act.code meaning "Past history" or "Family history" may substantially alter the interpretation of a [ <<404684003 | clinical finding ] and should not be used in this way. Instead the SNOMED CT context model should be used to capture these significant differences in meaning.

The approach to the use of Act.code in most Act class specializations is fairly straight forward. The exception to this is the Observation class where in some cases the way that the Observation.code and Observation.value attributes are populated and interpreted has led to extensive discussions which are summarized below.

A clinical record consists of statements related directly or indirectly to the health of a patient. Some statements relate to actions taken or requested as part of the provision of care. These actions may include procedures, investigations, referrals, encounters, supply and administration of medication. In the case of these statements, SNOMED CT expressions representing [ <<71388002 | procedure ] concepts provide appropriate content for the Act.code attribute of the relevant Act class specialization. Other statements in a clinical record relate to information found or derived in a variety of ways during the delivery of care. For the purpose, these statements can be referred to as “statements about clinical findings”. The way in which “statements about clinical findings” are represented has been a source of considerable discussion within HL7. This discussion focuses on the way in which the coded representation of such statements is expressed in the Observation.code” and Observation.value” attributes of the Observation class.

Statements about clinical findings can be divided into two categories.

A) Statements about findings in which two facets are clearly distinct

• (1) The action taken to make the finding (and/or the property about which the property was observed)
• (2) The result of the observation

Examples:

• Measurement of serum hemoglobin (1) = 14 g/dl (2)
  • This example follows the formal RIM semantics.
• Body weight (1) = 75 Kg (2)
  • This example is not in line with strict interpretation of the formal RIM definition in which the Observation.code is the action taken to make the observation. However, it is a more familiar form in real-world clinical statements about many observations. A possible bridge between these two views is to regard the name of the property observed (e.g. [ 27113001 | body weight ]) as implying the action to measure or observe that property (e.g. [ 39857003 | weighing patient ]).

B) Statements about findings that are often captured as a single “nominalized” expression

• The term "nominalized" is used to indicate a statement reduced to a single name (or term) which can then be coded as a single expression.
• The fact that a statement is often nominalized does not mean it consists of a single atomic item of information. Many such statements can be readily divided into several identifiable facets. However, unlike statements of type A, there are different views on how the semantics of the facets of these statements might be divided between the “code” and/or “value” attributes of the observation class.

Examples: The following examples are statements that might appear in clinical records. In each case they assert a finding in relation to the “record target”. Each of these examples illustrates a particular aspect of the potential for nominalizing a statement.

Record target …

• has a fracture of her left femur
• is complaining of pain in his right knee for the last two days
• reports that she had a heart attack in January 2001
• may have pernicious anemia
• has an aortic ejection murmur
• has normal visual acuity

Type (A) statements can be readily represented using the Observation class as documented in the RIM. However, a variety of options have been considered for type (B) "nominalized" statements. These options vary in the use they make of the Observation.code and Observation.value attributes.

In summary the options considered included

• Using only one of the attributes to represent the nominalized meaning of the statement and omitting the other attribute.
• Applying a fixed value to one of the attributes and using the other one to represent the nominalized meaning of the statement.
• Using the value to represent the nominalized meaning of the statement while allowing the other code to operate independently to track the question or process without affecting the meaning of the result to the observation.
• Creating a separate class for nominalized statement rather than using the Observation class.

As a result the preferred option is for a fixed Observation.code value "ASSERTION" to be used and for the meaning of the nominalized statement to be conveyed in the Observation.value. All other options are deprecated but some of these are permitted for backward compatibility.

The options permitted for backward compatibility are those that are known to be in use and these are supported as far as possible with transformation rules to allow the preferred form to be derived for comparison. There is a practical limitation to the transformation rules where code and value are used independently because it may not be possible to confirm computationally whether the code was intended to significantly modify the meaning of the value.

The values specified in the ActMood vocabulary partially overlap with SNOMED CT representations of [ 408729009 | finding context ] and [ 408730004 | procedure context ].

• SNOMED CT [ 408729009 | finding context ]:
  • Represents an assertion that the [ 246090004 | associated finding ] is: present, absent, a goal, a risk or an expectation.
  • May also represent an assertion that the presence or absence of a finding is unknown, possible or probable.
  • Applies to:
    • any SNOMED CT expression that represents a [ <<404684003 | clinical finding ].
    • any SNOMED CT expression that represents either a [ <<71388002 | procedure ] or an [ <<363787002 | observable entity ] provided that the expression is combined with a relevant result or value.
  • Is relevant to instances of HL7 Observation classes expressed in "event", "goal", "expectation" and "risk" moods.
• SNOMED CT [ 408730004 | procedure context ]
  • Represents an assertion that the [ 363589002 | associated procedure ] is: "requested", "planned", "started", "done", "cancelled", "not done", "not to be done" or one of several more specific [ <<408730004 | procedure context ] values.
  • May also represent an assertion that it is not known whether the procedure has been done.
  • Applies to any SNOMED CT expression that represents a [ <<71388002 | procedure ] (except where that expression is combined with a relevant result value).
  • Is relevant to:
    • instances of various HL7 Act classes including Procedure, SubstanceAdministration and Supply.
    • instances of the HL7 Observation class except in "intent" moods (including "request" and other subtype of "intent").

The following rules ensure validation and consistent interpretation of particular combinations of moodCode and SNOMED CT context. They also specify the context a particular moodCode value applies to a SNOMED CT expression that does not include an explicit representations of context.

1. The moodCode SHALL be present in all Act class instances⁴
2. If the code attribute of an instance of the Observation class, with a moodCode that is neither "intent" (INT) nor a subtype of "intent", is populated with a SNOMED CT expression, this expression MAY include an explicit representation of [ 408729009 | finding context ].
   • If the expression does not include an explicit [ 408729009 | finding context ], it SHALL be interpreted as having the default context specified for the relevant moodCode in Table 2.
   • If the expression includes an explicit [ 408729009 | finding context ], the context SHALL be compatible with the constraints specified for the relevant moodCode in Table 3. Any Act class instance does not conform to these constraints SHALL be regarded as an error.
3. If the value attribute of an instance of the Observation class is populated with a SNOMED CT expression, this expression MAY include an explicit representation of [ 408729009 | finding context ].
   • If the expression does not include an explicit [ 408729009 | finding context ], it SHALL be interpreted as having the default context specified for the relevant moodCode in Table 2.
   • If the expression includes an explicit [ 408729009 | finding context ], the context SHALL be compatible with the constraints specified for the relevant moodCode in Table 3. Any Act class instance does not conform to these constraints SHALL be regarded as an error.
4. If the code attribute of an instance of any Act class (except Observations included in points 2 or 3 above) is populated with a SNOMED CT expression, this expression MAY include an explicit representation of [ 408730004 | procedure context ].
   • If the expression does not include an explicit [ 408730004 | procedure context ], it SHALL be interpreted as having the default context specified for the relevant moodCode in Table 4.
   • If the expression includes an explicit [ 408730004 | procedure context ], the context SHALL be compatible with the constraints specified for the relevant moodCode in Table 5. Any Act class instance does not conform to these constraints SHALL be regarded as an error.
5. If a SNOMED CT expression includes an explicit statement of context this SHALL be validated by the rules stated above and SHALL be interpreted as a restatement or refinement of the meaning specified by the moodCode. The meaning of the SNOMED CT context SHALL NOT be interpreted as an independent compounding semantic modifier.
   

   For example
   moodCode="RQO" and code=[ 408730004 | procedure context | = 385644000 | requested ]
   This means "requested". It does not mean a "request to request".

   
   moodCode="INT" and code=[ 408730004 | procedure context | = 385650005 | organised ]
   This means "organized". It does not mean an "intention to organize".

   
   moodCode="INT" and value=[ 408729009 | finding context | = 410518001 | goal ]
   This is an error. It does not mean an "intention to set a goal".

Table 2 shows the mapping from moodCode to the default [ 408729009 | finding context ] for concepts that are subtypes of [ 404684003 | clinical finding ]. Table 4 shows the mapping from moodCode to default [ 408730004 | procedure context ] for concepts that are subtypes of [ <<71388002 | procedure ].

Table 3 shows the [ 408729009 | finding context ] validation constraints for SNOMED CT expressions based on the moodCode of the containing Act class instance. Table 5 shows the [ 408730004 | procedure context ] validation constraints for SNOMED CT expressions based on the moodCode of the containing Act class instance.

In these tables the symbol "<<" preceding a value indicates that either the value or any subtype of the value is permitted.

The context values in these tables are based on the following assumptions about other attributes in the same Act class instance:

• the HL7 negationInd is omitted from the Act class instance (see Act.negationInd (§ 2.2.9 ))
• the HL7 uncertaintyCode is omitted from the Act class instance (see Act.uncertaintyCode (§ 2.2.10 ))
• the HL7 statusCode in the Act class instance has a value that does not influence the context (see Act.statusCode (§ 2.2.4 ))

NOTE: For more information on statusCode dependent values see Table 7

For more information on statusCode dependent values see Table 7

Table 6 lists Act.moodCodes that have no direct relationship to SNOMED CT context attributes. While no constraints are specified for these moodCodes, some combinations may be irrational or open to misinterpretation. Therefore, caution should be used when combining these moodCodes with explicit representations of SNOMED CT context.

The Act.moodCode is a mandatory component all HL7 Act classes. Therefore this HL7 representation is required irrespective of whether SNOMED CT context representations are used.

SNOMED CT [ 408729009 | finding context ] and [ 408730004 | procedure context ] value hierarchies include more specific meanings than those associated with the Act.moodCode. Therefore, the SNOMED CT representation cannot be prohibited without resulting in loss of information.

For example, Act.moodCode cannot be used to express various:

• SNOMED CT [ 408730004 | procedure context ] values, including [ 410536001 | contraindicated ] and [ 385661002 | considered and not done ].
• SNOMED CT [ 408729009 | finding context ] values, including [ 410596003 | likely outcome ] and [410605003 | confirmed present ].

The SNOMED CT context model permits default context values to be applied, based on the surrounding information model. Therefore, inclusion of SNOMED CT context can be specified as optional, provided there are explicit rules (such as those in Table 2 and Table 4) for deriving default context values from the moodCode and, where relevant, from other HL7 Act class attributes.

The interaction between statusCode and SNOMED CT semantics varies according to the nature of the statusCode and the value of the Act.moodCode.

• The most general HL7 Act.statusCode values ("NORMAL", "OBSOLETE" and "NULLIFIED") relate to whether the Act class instance is currently valid. These states do not result in any overlaps with SNOMED CT semantics.
• Other states overlap with aspects of SNOMED CT semantics in a manner that is to some extent dependent on the mood of the Act.

Unlike the other attributes discussed in this section the value of the statusCode may progress over time. Thus the fact that a "request" was aborted implies that a request was made, as well as indicating that the request was not acted upon. Therefore, the impact of a Act.statusCode on SNOMED CT semantics depends on whether the concern is to know what steps were taken or to know whether a step was completed.

The relevance of statusCode is fairly clear cut when the Act.moodCode value is "EVN", since this implies an actual occurrence. In these cases, the statusCode pertains to whether the event is complete and thus directly to the SNOMED CT [ 408730004 | procedure context ].

In other moods, this relationship is less clear. For example, the Act.statusCode applies to an Act with moodCode "RQO" refers to the status of the request, whereas the [ 408730004 | procedure context ] refers to the progress of the concept specified by the [ 363589002 | associated procedure ].

The following rules deal only with cases where the Act.statusCode has a clear effect on the meaning of an Act class instance in a particular mood. Other rules or guidelines, based on similar principles, may be added in the future.

1. Act class instances SHALL be interpreted taking account of the Act.statusCode and the way particular values of this attribute when combined with the Act.moodCode may alter the default or permitted [ 408730004 | procedure context ] values.
2. In the case of an Act in "event" mood the defaults and constraints specified in (§ 2.2.3.2 ) and (§ 2.2.3.2 ) should be modified in accordance with statusCode as shown in Table 7.

The HL7 statusCode changes throughout the life cycle of an Act in its specified mood, until it reaches an end-state. Consideration of the impact of a statusCode on aspects of semantics depends on whether the requirement is to know 'what steps were taken' or 'whether a step was completed'. Thus the fact that a "request" was aborted implies that a request was made, as well as indicating that the request was not taken through to normal completion.

The statusCode values "new", "active", "held", "completed", "cancelled", "suspended", "nullified" and "obsolete" track the progress of the Act in its specified mood. The semantic relevance of statusCode in "event" mood is more clear cut than in other moods.

• For example, statusCode="completed"
  • when applied to an Act with moodCode="ENV" implies [ 408730004 | procedure context = 385658003 | done ]
  • when applied to an Act with moodCode="RQO" implies that the act of request has been completed. It does not mean that the requested action has been completed.

The statusCode values "NORMAL", "OBSOLETE" and "NULLIFIED" relate to the validity of a particular representation of an Act class instance. These states do not result in any overlaps with SNOMED CT semantics because the meaning of an Act class instance is no longer relevant if it has been "NULLIFIED" or marked as "OBSOLETE".

SNOMED CT finding concepts have a defining attribute that specifies the [ 363698007 | finding site ] and similarly SNOMED CT procedure concepts have a defining attribute that specifies the "procedure site". The post-coordination rules that apply to SNOMED CT permit refinement of these defining attributes. The resulting post-coordinated expressions can be represented in a single coded attribute using the HL7 Concept Description (CD) data type.

The result of this is that there are two completely overlapping approaches to the representation of sites associated with observations and procedures.

The following rules avoid redundancy and the risk of misinterpretation by restricting the use of targetSiteCode in Act class instances. There are two sections dealing with information models which 1) contain only SNOMED content and 2) allow multiple terminologies to be used.

If an Act.code or Observation.value contains only SNOMED-CT content then the following shall apply:

1. The targetSiteCode attribute SHOULD be omitted from any Act instance.
2. If necessary the specific site applicable SHOULD be represented as part of the SNOMED-CT expression (in Act.code or Observation.value) by refining the relevant site attribute as part of a pre post-coordinated expression.

If an Act.code or Observation.value contains SNOMED-CT content as one permitted code system then the following shall apply:

1. The targetSiteCode attribute SHALL be optional in any Act instance.
2. If the targetSiteCode attribute is present in an Observation or Procedure class instance in which the Act.code or Observation.value is expressed using SNOMED-CT then:
   • The targetSiteCode SHALL also be represented using SNOMED-CT
   • The targetSiteCode SHALL be the same as, or a subtype of, the value of the relevant site attribute as specified in the SNOMED-CT expression
   • The targetSiteCode SHALL be treated as equivalent to a restatement or refinement of the relevant site attribute in the SNOMED-CT expression
   • If the value of the targetSiteCode attribute is incompatible with the above rules then this SHALL be interpreted as an error

NOTE: The relevant site attribute depends on the SNOMED CT Concept Model for the type of procedure or finding. It may be one of the following: [ 363698007 | finding site ], [ <<363704007 | procedure site ], [ 405813007|procedure site - Direct ] or [ 405814001|procedure site - Indirect ]. In some cases, "procedure morphology", "direct morphology" or "indirect morphology" may also provide more specific site related information.

The notes following the definition of Observation.targetSiteCode make it clear that the intent is not to repeat a site implied by the Act.code.

Most observation target sites are implied by the observation definition and Act.code, or Observation.value. For example, "heart murmur" always has the heart as target. This attribute is used only when the observation target site needs to be refined, to distinguish right and left etc.

The notes following the Procedure.targetSiteCode definition are perhaps a little less clear cut. However, they convey a similar general sense.

Some target sites can also be "pre-coordinated" in the Act definition, so that there is never an option to select different body sites. The same information structure can handle both the pre-coordinated and the post-coordinated approach.

Therefore, if the Procedure.code or Observation.code specifies the site to a sufficient level of detail, there is no requirement to include a separate targetSiteCode attribute. When using SNOMED CT post-coordination to refine the site, the Act.code specifies the site to the same level of detail as can be achieved using the targetSiteCode.

SNOMED CT offers additional features which make it significantly more expressive than the targetSiteCode:

• Specific subtypes of the [ <<363704007 | procedure site ] attribute distinguish between the direct and indirect targets of a procedure. One use of this is to ensure that removal of a something from an organ does not classify as a type of removal of that organ.
  • For example:
    • [ 287742007|ureter calculus removal ]
      is defined as a procedure with
      [ {260686004|method|=129303008|removal - action|
      ,363700003|direct morphology|=56381008|calculus|
      ,405814001|procedure site - Indirect|=87953007|ureteric structure|} ]
    • [ 51607004|total ureterectomy | ]
      is defined as a procedure with ...
      [ {260686004|method|=129304002|excision - action|
      ,405813007|procedure site - Direct|=302511008|entire ureter|} ]
• Explicit grouping of attributes allows representation of multiple sites associated different actions in a single procedure.
  • For example
    • The procedure [ 11401008 | dilation and curettage of uterus ] involves dilatation of one site (cervix uteri) and curettage of another (endometrium) so it is defined as a procedure with the following two relationship groups ...
    • [ {260686004 | method | = 129319001 | curettage - action |
      ,405813007 | procedure site - Direct | = 2739003 | endometrial structure | }
      {260686004 | method | = 129419002 | dilation - action |
      ,405813007 | procedure site - Direct | = 71252005 | cervix uteri structure | } ]

The recommendation to use the SNOMED CT representation of site is based on the added expressivity. Omission of the HL7 targetSiteCode attribute is recommended to avoid the redundancy and the potential for conflicts between the two forms of representation of site. However, while the use of these HL7 attributes is deprecated, it is permitted to support use in environments that do not support SNOMED CT post-coordination. In this case, requiring these attributes to be encoded using SNOMED CT and interpreted as refinements of the relevant SNOMED CT attributes, enables a simple transformation to the recommended form.

SNOMED CT procedure concepts have a defining attribute that specifies the [ 424876005 | approach ] and has a comparable meaning. The post-coordination rules that apply to SNOMED CT permit refinement of this defining attribute. The resulting post-coordinated expressions can be represented in a single coded attribute using the HL7 Concept Description (CD) data type.

The result of this is that there are two completely overlapping approaches to the representation of approaches associated with procedures.

While HL7 models SubstanceAdministration as a separate class from Procedure, the SNOMED CT concept [ 432102000 | administration of therapeutic substance ] is a subtype of procedure. Therefore the [ 424876005 | approach ] attribute can also be applied to refine SNOMED expressions that encode the action associated with SubstanceAdministration. Therefore, this overlap also applies to that class.

The following rules avoid redundancy and the risk of misinterpretation by restricting the use of the approachSiteCode in Procedure and SubstanceAdministration class instances. There are two sections dealing with information models which 1) contain only SNOMED content and 2) allow multiple terminologies to be used.

If a Procedure.code or SubstanceAdministration.code contains only SNOMED-CT content then the following shall apply:

1. The approachSiteCode attribute SHOULD BE omitted from any Act instance.
2. If necessary the specific site applicable SHOULD be represented as part of the SNOMED-CT expression (in Procedure.code or SubstanceAdministration.code) by refining the relevant site attribute as part of a pre or post-coordinated expression.

If a Procedure.code or SubstanceAdministration.code contains SNOMED-CT content as one permitted code system then the following shall apply:

1. The approachSiteCode SHALL be optional in any Act instance.
2. • The approachSiteCode SHALL also be represented using SNOMED-CT
   • The approachSiteCode SHALL be the same as, or a subtype of, the value of the relevant site attribute as specified in the SNOMED-CT expression
   • The approachSiteCode SHALL be treated as equivalent to a restatement or refinement of the relevant site attribute in the SNOMED-CT expression
   • If the value of the approachSiteCode attribute is incompatible with the above rules then this SHALL be interpreted as an error

The notes following the Procedure.approachSiteCode definition suggest that the intent is not to repeat the approach if it is fixed by the nature of the procedure specified by the Act.code.

Some [ 424876005 | approach ] sites can also be "pre-coordinated" in the Act definition, so that there is never an option to select different body sites. The same information structure can handle both the pre-coordinated and the post-coordinated approach.

Therefore, if the Procedure.code or SubstanceAdministration.code specifies the [ 424876005 | approach ] to a sufficient level of detail, there is no requirement to include a separate approachSiteCode attribute. When using SNOMED CT post-coordination to refine the [ 424876005 | approach ] , the Act.code can specify the approach to the same level of detail as can be achieved using the approachSiteCode attribute.

The vocabulary domain specified for approachSiteCode is ActSite which is the same as the vocabulary domain for targetSiteCode. In contrast SNOMED CT uses a specific value hierarchy for approaches which is different from the one used for [ 363698007 | finding site ] or [ <<363704007 | procedure site ]. The distinction is that an approach is a route used to reach a target site rather than a specific structural landmark that represents a point on or part of that route.

The example values in the approachSiteCode include a mixture of approaches (e.g. "trans-abdominal approach" and "retroperitoneal approach") which fit the idea of approach as used by SNOMED CT. However, references to the punctured area of skin or structural landmarks have a significantly different semantic quality. Many sites are never the names of routes, several routes may pass through a single site and a route may pass through several sites. Therefore attempts to combine SNOMED CT and HL7 representations of approach may result in confusion rather than clarity.

The recommendation to use the SNOMED CT representation of [ 424876005 | approach ] is based on the more appropriate range of values available for this attribute, and on the fact that many procedure concepts pre-coordinate an implied or explicitly stated approach. Omission of the HL7 approachSiteCode attribute is recommended to avoid the redundancy and the potential for conflicts between the two forms of representation of site. However, while the use of these HL7 attributes is deprecated, it is permitted to support use in environments that do not support SNOMED CT post-coordination. In this case, requiring the approachSiteCode attributes to be encoded using SNOMED CT and interpreted as refinements of the SNOMED CT [ 424876005 | approach ] , enables a simple transformation to the recommended form.

SNOMED CT concepts have a defining attribute that specifies the [ 260686004 | method ] used. SNOMED CT "evaluation procedure" concepts, which may be used to specify the nature of an observation, have a defining attribute that specifies the [ 370129005 | measurement method ]. SNOMED CT [ 404684003 | clinical finding ] concepts, which may be used as values of a nominalized observation or assertion, have a defining attribute that specifies the [ 418775008 | finding method ]. The post-coordination rules that apply to SNOMED CT permit refinement of this defining attribute. The resulting post-coordinated expressions can be represented in a single coded attribute using the HL7 Concept Description (CD) data type.

The result of this is that there are two overlapping approaches to the representation of methods associated with observations and procedures.

The following rules avoid redundancy and the risk of misinterpretation by restricting the use of the methodCode in Procedure and Observation class instances. There are two sections dealing with information models which 1) contain only SNOMED content and 2) allow multiple terminologies to be used.

If an Act.code or Observation.value contains only SNOMED-CT content then the following shall apply:

1. The methodCode attribute SHOULD be omitted from any Act instance.
2. If necessary the method applicable SHOULD be represented as part of the SNOMED-CT expression (in Act.code or Observation.value) by refining the relevant method attribute as part of a post-coordinated expression.

If an Act.code or Observation.value contains SNOMED-CT content as one permitted code system then the following shall apply:

1. The methodCode attribute SHALL be optional in any Act instance.
2. If the methodCode attribute is present in an Observation or Procedure class instance in which the Act.code or Observation.value is expressed using SNOMED-CT then:
   • The methodCode SHALL also be represented using SNOMED-CT
   • The methodCode SHALL be the same as, or a subtype of, the value of the relevant method attribute as specified in the SNOMED-CT expression
   • The methodCode SHALL be treated as equivalent to a restatement or refinement of the relevant method attribute in the SNOMED-CT expression
   • If the value of the methodCode attribute is incompatible with the above rules then this SHALL be interpreted as an error

NOTE: The relevant method attribute depends on the SNOMED Concept Model in respect of the type of procedure or finding. It may be one of the following: [ 260686004 | method ], [ 418775008 | finding method ] or [ 370129005 | measurement method ].

The notes following the definition of Observation.methodCode make it clear that the intent is not to repeat a method implied by the Act.code.

In all observations the method is already partially specified by simply knowing the kind of observation (observation definition, Act.code) and this implicit information about the method does not need to be specified in Observation.methodCode.

The notes following the Procedure.methodCode are less explicit about avoidance of duplication. However, they do suggest that code systems might be designed with relationships between procedures and possible method – which is exactly how SNOMED CT is designed. What the note does not take into account is that the terminology may also specify a way to represent a specific method with the procedure in a single code or expression.

'… a code system might be designed such that it specifies a set of available methods for each defined Procedure concept'

Therefore, if the Act.code or Observation.value specifies the method to a sufficient level of detail, there is no requirement to include a separate methodCode attribute. When using SNOMED CT post-coordination to refine the method, the Act.code or Observation.value specifies the method to the same level of detail as can be achieved using the methodCode.

The notes on methodCode use "open" and "laparoscopic" procedures as examples of differences in method. SNOMED CT makes this same differentiation using another defining attribute [ 260507000 | access ]. This highlights the potential for confusion from using both SNOMED and HL7 representations of method.

The semantics of this attribute potentially overlaps with SNOMED CT “[ 260870009 | priority ]” attribute which "... refers to the priority assigned to a procedure".

The following rules recommend specific uses for the HL7 and SNOMED CT representations of priority. There are two sections dealing with information models which 1) contain SNOMED content and 2) allow multiple terminologies to be used.

In all cases:

1. Act class clones SHALL include the priorityCode attribute if there is a requirement for expressing the urgency of a request, tracking and auditing services based on requested prioritization or any other aspects of workflow management related to priority.
2. Act class clones need not include the priorityCode attribute if the only requirement for indicating priority relates to distinguishing differences between the nature of the procedure itself based on priority (see example below).
3. In Act class instances the Act.priorityCode attribute SHALL be used where it has a specific functional role in relation to the purpose of a communication.
   • For example, to prioritize a request for a service or to track the priority under which a service was scheduled and carried out.

In cases where SNOMD-CT is used.:

1. In Act class instances, the relevant SNOMED CT priority attribute SHOULD be included in the expression in the Act.code or Observation.value if the priority significantly alters the nature of the statement.
   • The priorityCode SHALL be the same as, or a subtype of, the value of the relevant priority attribute as specified in the SNOMED-CT expression
   • For example, an [ 274130007 | emergency cesarean section ] is a significantly different procedure when compared with an [ 177141003 | elective caesarean delivery ].

At face value the Act.priorityCode and the SNOMED CT [ 260870009 | priority ] attribute appear to have similar meanings. However, the way in which these are used appears to differ significantly.

• The SNOMED CT attribute is used to specify the defining characteristic that distinguishes an elective procedure from an emergency procedure. Like any defining attribute it can also be used to refine or qualify a procedure that is specified without references to its urgency.
• The HL7 priorityCode is generally used to communicate priority in relation to workflow management and audit. One obvious use case for this is to allow request for a service to indicate the priority assigned to it by the requester.

The use of a distinct information model attribute (i.e. Act.priorityCode), which is applicable to all services, makes the priority more readily accessible to workflow management. It does not require the SNOMED CT expression to be parsed to determine the priority of a request or action. In addition, it allows consistent handling of priority when some services are represented using SNOMED CT while others are represented using different code systems.

The use of a representation of priority that is integrated with the definition model of the associated concept is more useful from the perspective of clinical record retrieval. The description logic model of SNOMED CT ensures the computational equivalence of a procedure concept defined as an emergency and a post-coordinated expression in which [ 260870009 | priority | = 25876001 | emergency ] is added to the more general procedure concept.

The semantics of this attribute overlaps with:

• SNOMED “[ 408729009 | finding context ]” values indicating absence of a specified finding.
• SNOMED CT “[ 408730004 | procedure context ]” values indicating that a specified procedure was not done.

This overlap leads to a potential ambiguity since a combination of negationInd with a contextual representation of absence might be interpreted either as:

• double negation (i.e. "finding X is not absent" which is equivalent to "finding X is present")
• restatement or emphasis of the negative resulting from a mapping between the two ways to indicate negation or absence (i.e. "negative observation: finding X is absent" which means "finding X is absent").

The following rules avoid the risk of misinterpretation by prohibiting use of the negationInd in Act class instances that are encoded using SNOMED CT.

1. In a constrained information model or template the negationInd attribute SHOULD be omitted from:
   • any Act class clone in which SNOMED CT is the only permitted code system for the Act.code attribute.
   • any Observation class clone in which SNOMED CT is the only permitted code system for the Observation.value attribute.
2. In a constrained information model or template, the negationInd attribute SHALL be optional if it is included in:
   • an Act class clone in which SNOMED CT is one of the permitted code systems for the Act.code attribute.
   • any Observation class clone in which SNOMED CT is one of the permitted code systems for the Observation.value attribute.
3. The negationInd attribute SHOULD be omitted from any Act class instance in which the Act.code attribute is expressed using SNOMED CT.
   • Negative assertions about an Act (e.g. "procedure not done", "substance not to be administered" ) SHOULD be represented, as part of the SNOMED CT expression in the Act.code attribute, by including the appropriate explicit [ 408730004 | procedure context ] (e.g. [ 3385660001 | not done ] or [410521004 | not to be done]).
4. The negationInd attribute SHOULD be omitted from any Observation class instance in which the Observation.value attribute is expressed using SNOMED CT.
   • Assertions of negative Observations (e.g. absence of a specified finding) SHOULD be represented, as part of the SNOMED CT expression in the Observation.value attribute, by including the appropriate explicit [ 408729009 | finding context ].
5. The negationInd attribute MAY be included in an Act class instance where the SNOMED CT expression represents a [ <<71388002 | procedure ] with no explicit [ 408730004 | procedure context ] or a [ <<129125009 | procedure with explicit context ] with [ 408730004 | procedure context | = 385658003 | done ]
   • This approach is not recommended but is permitted to allow simple negation in systems that do not support the SNOMED CT context model. If it is used, it SHALL be interpreted as equivalent to the specified [ 363589002 | associated procedure ] with [ 408730004 | procedure context | = 3385660001 | not done ].
6. The negationInd attribute MAY be included in an Observation class instance where the SNOMED CT expression represents a [ <<404684003 | clinical finding ] with no explicit [ 408729009 | finding context ] or a [ <<413350009 | finding with explicit context ] with the [ 408729009 | finding context = 410515003 | known present ].
   • This approach is not recommended but is permitted to allow simple negation in systems that do not support the SNOMED CT context model. If it is used, it SHALL be interpreted as equivalent to the specified [ 246090004 | associated finding ] with a [ 408729009 | finding context | = 410516002 | known absent ].
7. If the negationInd attribute is present in an Act class instance in which either the Act.code or Observation.value is expressed using SNOMED CT, it SHALL be interpreted as an error unless the conditions noted in points 5 and 6 above apply.

The Act.negationInd is an optional RIM attribute which negates the meaning of an Act. This negation is unnecessary in cases where SNOMED CT is used because the context attributes can be used to specify the absence of a finding or the fact that a procedure has not been done. Including both representations introduces potential for serious misinterpretation of combinations including the following:

• Double negative
  • If negationInd is true and the SNOMED CT [ 408729009 | finding context | = 410516002 | known absent ] the double negative would be “not known absent” (i.e. “present”).
  • If negationInd is true and the SNOMED CT [ 408730004 | procedure context | = 3385660001 | not done ] the double negative would be “not not done” (i.e. “done”).
  • For the avoidance of potential ambiguity this option is explicitly prohibited by rules in this document.
• Indication or emphasis of negation
  • HL7 negationInd indicates the presence of negation and the SNOMED CT context provides more details of the nature of the negation.
  • Implies that if negationInd is true and the Act is coded with SNOMED CT an appropriate negated SNOMED CT finding or procedure context value (e.g. [ 410516002 | known absent ] or [ 3385660001 | not done ]) should also apply.
  • Might imply that if a negated SNOMED CT finding or procedure context value (e.g. [ 410516002 | known absent ] or [ 3385660001 | not done ]) is applied the negationInd should be true.
• Restatement of negation
  • HL7 negationInd and SNOMED CT negative contexts apply as alternatives and when combined serve to restate the negation
  • Implies that if only negationInd is present a mapping table is required to the relevant SNOMED CT context to enable consistent interpretation. This mapping table would need to specify combinations of moodCode and negationInd. For example, negationInd=true with moodCode=“EVN” the would imply [ 408730004 | procedure context | = 385660001 | not done ], whereas negationInd=true with moodCode=“RQO” might imply [ 408730004 | procedure context | = 410521004 | not to be done ].

The simplest way to avoid these possible misunderstandings is to omit the negationInd attribute and allow the SNOMED CT context attributes to provide information about absent findings and procedures that have not been done. However, to meet requirements to support some simple negation in systems that do not support the SNOMED CT context model, use of negationInd is permitted where it cannot be misinterpreted . The only cases where no risk of misinterpretation are where the SNOMED CT context is either unspecified or explicitly states the default values [ 410515003 | known present ] or [ 385658003 | done ]. The negationInd can be used to switch these defaults to the appropriate negated values such as [ 410516002 | known absent ] and [ 385660001 | not done ].

The semantics of this attribute overlaps with SNOMED CT [ 408729009 | finding context ] values [ <<410590009 | known possible ] and its subtypes including [ 410592001 | probably present ] and [ 410593006 | probably not present]. This provides different ways to express the uncertainty of a finding and ambiguity about the impact of combining these two representations in a single Act class instance.

The following rules avoid the risk of misinterpretation by prohibiting use of the uncertaintyCode in Act class instances that are encoded using SNOMED CT. There are two sections dealing with information models which 1) contain only SNOMED content and 2) allow multiple terminologies to be used.

If an Act.code or Observation.value contains only SNOMED-CT content then the following shall apply:

1. The uncertaintyCode attribute SHOULD be omitted from any Act instance.
2. If necessary the uncertainty applicable SHOULD be represented as part of the SNOMED-CT expression by refining the relevant context attribute as part of a post-coordinated expression.

If an Act.code or Observation.value contains SNOMED-CT content as one permitted code system then the following shall apply:

1. The uncertaintyCode attribute SHALL be optional in any Act instance.
2. If the uncertaintyCode attribute is present in an Act class instance in which the Act.code or Observation.value is expressed using SNOMED-CT then:
   • The uncertaintyCode SHALL also be represented using SNOMED-CT
   • The uncertaintyCode SHALL be the same as, or a subtype of, the value of the relevant context attribute as specified in the SNOMED-CT expression
   • The uncertaintyCode SHALL be treated as equivalent to a restatement or refinement of the relevant context attribute in the SNOMED-CT expression
   • If the value of the uncertaintyCode attribute is incompatible with the above rules then this SHALL be interpreted as an error

The SNOMED CT [ 408729009 | finding context ] values provide a more specific way to express uncertainty about the presence or absence of a finding. This is therefore preferred over the use of the optional uncertaintyCode attribute.

The SNOMED CT [ 408730004 | procedure context ] does not contain values to represent "possibly done" or "probably done". As a result there is no obvious way to express uncertainty about whether a procedure has been done. This may be relevant if an informer reports something like "I think I had a tetanus vaccination but I am not sure". The current advice is to treat such information as an Observation about past history, rather than adding uncertainty value to the [ 408730004 | procedure context ] value hierarchy. However, this issue has been raised with the SNOMED Concept Model Working Group and the advice may be revised if after further consideration the [ 408730004 | procedure context ] value set is expanded.

The HL7 UVP (Uncertain Value - Probabilistic) data type was considered as this as another HL7 approach to representation of uncertainty. The UVP data type is defined as "A generic data type extension used to specify a probability expressing the information producer's belief that the given value holds." The data types specification adds that "How the probability number was arrived at is outside the scope of this specification." There is some potential for overlap as the UVP data type is a "generic data type extension". This means it can be applied to any other data type, and hence to any HL7 attribute. This data type may be applied to attribute values associated with a SNOMED CT code. For example, to express uncertainty associated with the value of a particular measurement. However, use of UVP to apply a specific level of uncertainty to a SNOMED CT concept in an Act should be avoided.

There are multiple scenarios that may result in overlap, particularly with data recorded in Observation.code or Observation.value.

1. Within HL7 v3
   • From a modeling perspective, the “interpretation” of a value (i.e. laboratory test result, etc.) is an additional “higher level” observation that is made on the results (raw data) from a particular investigation. This additional observation can, and likely in some cases does, carry the full semantics of an “observation” (including author, author time, etc.), and thus may be represented by an additional instance of an Observation class. Alternatively, and more commonly, the result of this additional observation is represented using the interpretationCode attribute within the original Observation class (which is essentially a lightweight or “shortcut” method for representing this data). There is the potential for conflict and/or ambiguity between these two approaches.
   • In some situations, current practice in V3 laboratory messaging (parallel to the common usage in V2) has been to use the Observation.interpretationCode in place of, rather than in conjunction with Observation.value. This typically occurs in laboratory antimicrobial susceptibility messaging (see the Streptococcus susceptibility to Penicillin G example above).
2. Between HL7 v3 and SNOMED CT
   • The SNOMED-CT concept model for Clinical Findings provides the [ 363714003 | Interprets ] and [ 363713009 | Has interpretation ] attributes. The latter represents similar notions to those intended for use by the HL7 Observation.interpretationCode attribute. An example of the use of these attributes is:
     
     [ 165558001 | Platelet count abnormal ]
     
     Includes, in its reference definition:
     
     363714003 | Interprets |=61928009 | Platelet count |,
     363713009 | Has interpretation |=263654008 | Abnormal |
   • Whether primitively represented or defined according to the above scheme, SNOMED CT contains many pre-coordinated ‘Finding’ concepts, that include notions similar to those expressed as ‘interpretations’, for example:
     
     [ 110368006 | Decreased estrogen level]
     [ 102659003 | Normal glucose level ]
     
     It is therefore possible to represent, using a SNOMED CT ‘Finding’ in Observation.value, notions such as ‘normality’, ‘abnormality’, ‘resistance’.

Given the complexities described in ‘discussion and rationale’, it is not currently possible to provide normative guidance on the use of Observation.interpretationCode. In particular, it is not possible to provide guidance on the prohibition of Observation.interpretationCode where SNOMED CT is the only permitted code system for the Act.code.

However, the following guidance (with caveats) can be provided:

• In a constrained information model or template that permits or requires the use of SNOMED CT to represent the nature of an Observation class clone, or in which SNOMED CT is one of the permitted code systems for the Observation.value attribute, Observation.interpretationCode SHALL be optional.
• In any Observation class instance in which the Act.code or Observation.value attribute is expressed using SNOMED CT, and Observation.interpreationCode is present, it shall take its value from the following ranges in SNOMED CT:
  
  ((<<281296001 | result comments |) OR (<<260245000 | findings values |))
  
• Unless explicitly specified by model designers or communicating parties, SNOMED CT findings that represent ‘interpretation’ notions are not prohibited from use. It is possible, therefore, that receiving systems and analytic queries that wish to detect ‘interpretation’ notions may also need to test the SNOMED CT concept carried as Observation.value.

Relevant to this topic, an HL7 Observation will currently support the representation of three notions:

• The thing being observed (in Observation.code)
• The result of the observation (in Observation.value)
• The interpretation of the result of the observation (in Observation.interpretationCode)

Either primitively represented or modeled using the ‘has interpretation’ attribute, SNOMED CT will support the representation of the following notions:

• The thing being observed (in Observation.code)
• The thing being observed and interpretation of the result of the observation (in Observation.value)

There is therefore incomplete overlap in ‘interpretation’ representation, and incomplete expressivity of SNOMED CT to support all aspects of representation (a SNOMED CT Expression cannot exhaustively communicate the result of an observation and its interpretation).

Evidence suggests that Observation.interpretationCode is currently used, and it is not possible currently to provide a SNOMED CT-only representation to allow its prohibition.

Neither is it possible, currently, to enhance normalization rules to support equivalence detection between ‘interpretations’ communicated in Observation.value or in Observation.interpretationCode.

SNOMED CT contains concepts that represent most of the widely used units and these overlap with the UCUM representation. These SNOMED CT concepts could be represented in the translation sub-element of the PQ datatype. However, this would introduce redundancy and the potential for conflict between the alternative representations.

The following guidance is intended to reduce the need for redundant representation of units and maximize the opportunity for automated unit conversion.

1. Wherever possible the unit element of the HL7 PQ (physical quantity) data type SHOULD be encoded using the appropriate UCUM representation and not using a SNOMED CT concept identifier⁶.
2. In the case of informal units, which have no standard UCUM representation, a SNOMED CT concept identifier MAY be used in the translation sub-element of the unit element.
   • Examples of informal units include "capsules" or "tablets". In these case the unit "1" (the UCUM symbol meaning "the unity") SHOULD be used and the SNOMED CT representation of the nature of the counted unit MAY then be used.

Use of UCUM representation simplifies interoperability using HL7 messages. The UCUM specification also supports translation between different types of units. It is possible to map from SNOMED CT concepts to UCUM in all cases except those where an informal unit is specified. On the other hand, since the UCUM representation is an expression syntax it can be used to represent an almost unlimited range of complex units in a formal mathematical manner. Many of the units that can potentially be represented in UCUM have no pre-coordinated equivalent in SNOMED CT. SNOMED post-coordination does not currently support the type of mathematical formalism that UCUM offers.

The SNOMED CT [ 408731000 | temporal context ] distinguishes between findings or procedures that are recorded as part of "past" history and those that are recorded as [ 15240007 | current ]. It also allows a distinction to be made between a specified point or period in time (e.g. and a more general unspecified time (e.g. [ 410588008 | past - unspecified ]). The [ 408731000 | temporal context ] potentially affects the interpretation HL7 date and time attributes.

When a SNOMED CT expression (or concept definition) includes an explicit representation of [ 408731000 | temporal context ], the effectiveTime might be interpreted either as "the time at which the situation applied" or "the time at which the focus concept applied". Guidance is needed to avoid this potentially misleading ambiguity.

• For example, the definition of the concept [407553003 | history of - glandular fever] includes:
  • [ 246090004 | associated finding | = 271558008 | glandular fever | , 408731000 | temporal context | = 410513005 | past ]
    • The concept [407553003 | history of - glandular fever] represents a situation.
    • The value of the associated finding attribute is the focus concept (i.e. the concept [ 271558008 | glandular fever ]).
  • When an Observation asserts the value [407553003 | history of - glandular fever], the effectiveTime might be interpreted as:
    • The time when the focus concept applied (i.e. the time in the past when they actually had glandular fever);
    • The period of time during which the situation applied (i.e. the period of time during which they had a "history of glandular fever" (i.e. a period of time from when they first had glandular fever with no upper bound or extending until death);
    • The time during an episode of care when the situation was recognized (i.e. a period starting when "history of glandular fever" was first recorded as part of the record of this episode of care);
    • The time during which the situation was considered clinically relevant (i.e. a period of time based on a clinical judgment of the significance of a past history of glandular fever to the currents longer term health).

The following rules clarify the impact of [ 408731000 | temporal context ] on interpretation of HL7 data and time attributes associated with an Act class instance.

1. If a SNOMED CT expression includes an explicit [ 408731000 | temporal context ] value, the effectiveTime SHALL be interpreted as applying to the time at which the focus concept applied.
   • The focus concept is the value of the [ 246090004 | associated finding ] or [ 363589002 | associated procedure ] in the SNOMED CT expression or concept definition.
     • For example, the Act.effectiveTime for [ 407553003 | history of - glandular fever ] is the time, in the past, when they had glandular fever.
   • If the [ 408729009 | finding context ] indicates negation (e.g. [ 408729009 | finding context | = 410516002 | known absent ]), the Act.effectiveTime refers to the point in time or period or time during which the focus concept was known to be absent
   • Similarly, if the [ 408730004 | procedure context ] has a negative value such as [ 3385660001 | not done ] the Act.effectiveTime refers to the point in time or period or time during which the focus concept was not done.
     • For example, the Act.effectiveTime for [ 165139002 | endoscopy not carried out ] represents the time at which, or period during which, an endoscopy was not done.
2. If the SNOMED CT expression in an Act class instance specifies [ 408731000 | temporal context | = ([ 410584005 | current - specified |) OR ( 410587003 | past - specified )]:
   • the Act.effectiveTime attribute SHALL be present and its value SHALL be interpreted as the clinically relevant point or period in time at which the focus concept applied or is intended to apply.
3. If the SNOMED CT expression in an Act class instance does not explicitly specify [ 408731000 | temporal context ] or explicitly specifies [ 408731000 | temporal context | = ( 410512000 | current or specified | ) OR ( 15240007 | current | ) OR ([ 410585006 | current - unspecified |)]:
   • the Act.effectiveTime attribute SHOULD be included and, if present, its value SHALL be interpreted as the clinically relevant point in time or period during which the associated finding procedure applied or is intended to apply.
     • If the Act.effectiveTime represents a period of time with an upper bound either set in the future or omitted, this indicates that the focus concept continues (or is expected to continue) to apply beyond the time when it was recorded.
   • If the Act.effectiveTime attribute is omitted (or contains a null flavor), the Participation.time value stated for a performer MAY be regarded as an approximation to the clinically relevant time.
4. If the SNOMED CT expression in an Act class explicitly specifies [ 408731000 | temporal context = ((410513005 | past |) OR ( 6493001 | recent |))]:
   • the Act.effectiveTime attribute MAY be included and, if present, its value SHALL be interpreted as the clinically relevant point or period in time to which the focus concept applied or is intended to apply.
   • the Participation.time value stated for an author SHALL be regarded as the time at which it was asserted that this procedure or observation was carried out in the past.
5. If the SNOMED CT expression in an Act class instance explicitly specifies [ 408731000 | temporal context | = 410588008 | past - unspecified ]:
   • the Act.effectiveTime attribute SHALL NOT be included as this would contradict the asserted [ 408731000 | temporal context ].
   • the Participation.time value stated for an author SHALL be interpreted as the time at which it was asserted that this procedure or observation was carried out in the past.
6. If the SNOMED CT expression in an Act class explicitly specifies [ 408731000 | temporal context | = 410589000 | all times past ]:
   • the Act.effectiveTime attribute SHOULD NOT be included but, if present, it SHALL only specify the upper bound of a period of time.
     • Note: The [ 408731000 | temporal context | = 410589000 | all times past ] is used with [ 408729009 | finding context | = 410516002 | known absent ] to assert a negative past history or a negative family history (e.g. [ 266882009 | no family history or ischemic heart disease ]). Negative assertions of this type imply that at [ 410589000 | all times past ] the focus concept did not apply. It is reasonable to combine this with the upper bound of a period of time, as the finding may be true at some future point in time (e.g. the patient may now be diagnosed as having asthma, although they have no past history of asthma). However, it would be contradictory to specify a point in time value or a lower bound for a period of time.
   • the Participation.time value stated for an author SHALL be interpreted as the time at which it was asserted that at all times past this Observation applied.

In most cases, following the general rules specified by the HL7 RIM allow unequivocal interpretation of the meaning of the Act.effectiveTime and associated Participation.time values. However, there are several possible interpretations of Act.effectiveTime, in relation to a SNOMED CT expression which includes an explicit past history temporal context (i.e. [ 408731000 | temporal context << 410513005 | past ]). Therefore, the rules specified above require that the relative time as specified by the SNOMED CT [ 408731000 | temporal context ] and any specific point or period of time expressed in Act.effectiveTime should be consistent with one another. The rules do not permit the effectiveTime and [ 408731000 | temporal context ] to be interpreted in a combinatorially manner. Thus if the [ 408731000 | temporal context | = 410513005 | past ] the Act.effectiveTime, if stated, must be the point or period in the past when the finding applied.

SNOMED CT includes qualifiers that allow refinement of meaning using post-coordinated expressions. As a result, the use of an additional Observation class is unnecessary and introduces alternative ways to represent the same meaning.

The following rules are specified to simplify interpretation by minimizing unnecessary variability in representation.

1. A constrained information model or template that permits use of SNOMED CT as one of the permitted ways to represent the result of an Observation, MAY include related Observation classes included to permit qualification of the meaning of an Observation but inclusion of these qualifying class SHALL NOT be required.
2. A constrained information model or template that requires use of SNOMED CT to represent the result of an Observation SHALL NOT include any related Observation classes included to permit qualification of the meaning of an Observation.
3. An Observation class instance in which the Observation.value is represented by a SNOMED CT expression SHALL NOT include any related qualifying classes but SHOULD encode the relevant qualifications as part of the expression.

It is important to reduce the scope for unnecessary alternative representation of the same information. Tight coupling of the qualification to the primary result of the observation is likely to reduce the risk of misinterpretation.

SNOMED CT provides a variety of attributes that can be used to represent relationships between different concepts in a post-coordinated expression. These post-coordinated expressions have the potential to represent some association that might alternatively be represented using ActRelationships. For example, the attributes [ 42752001 | due to ] could be used to construct expressions such as [ 49218002 | hip pain |: 42752001 | due to | = 396275006 | osteoarthritis ], which could also be represented using two separate Observations linked by an ActRelationship with the typeCode "caused by".

There is no absolute rule about when to express linkage in the terminology and when to use linkage mechanisms in the RIM (e.g. ActRelationships). However, the following guidance should be followed:

• A single identifiable observation, assertion or procedure SHOULD usually be represented by a single Act class instance containing an appropriate SNOMED CT expression.
• A single Act class instance SHOULD be used to represent an integral combination of facets applicable to a single identifiably observation, assertion or procedure. Some examples of integral combination are shown below. The common feature of these is that together they represent a finding with a distinct pattern and a shared life history.
  • A combination of findings is a part of a single recognizable condition
    • E.g. "Headache preceded by visual disturbance".
  • A disorder is specialized by a specific cause
    • E.g. "Pneumonia due to streptococcus pneumoniae".
  • The nature of a disorder is determined by another condition
    • E.g. [ 4855003 | diabetic retinopathy ].
  • A temporal or causative relationship between two concepts in which one is a specific symptom or diagnostic criterion for the other.
    • E.g. [ 51771007 | postviral fatigue syndrome ], "Shortness of breath after moderate exercise".
  • A single recognized procedure involves two or more distinct but related actions:
    • E.g. [86477000 | total hysterectomy with removal of both tubes and ovaries ], "Reduction and fixation of a fracture"
• Post-coordinated SNOMED CT expressions SHOULD NOT be used to artificially combine distinct observations, assertions and procedures into a single Act class instance.
  • The line between integral combinations of items and distinct items is not clear-cut. However, as a general rule two items SHOULD be considered to be distinct if
    • they are capable of being independently validated (i.e. the accuracy of one statement is not dependent on the accuracy of the other)
    • their life histories differ and are independent of one another
    • the relationship between them is a matter of judgment rather than fact
• Distinct observations, assertions and procedures SHOULD be represented by separate Act class instances related to one another by appropriate ActRelationships.
  • Multiple distinct findings in a patient that may or may not be associated with one another or with some more general problem.
    • E.g. A collection such as "chest pain" with "shortness of breath" finding of "tachycardia" and "ECG abnormality" interpreted as "Myocardial infarction".
  • Multiple conditions occur contemporaneously (or in sequence) where the nature of individual conditions is specific to the presence of the other condition.
    • E.g. "AIDS" and "gastro-enteritis"
  • Multiple distinct procedures incidentally performed at the same time or during the same hospital stay.

In general SNOMED CT expressions (whether pre-coordinated or post-coordinated) are most appropriate for expressing multiple facets of a single logical concept. On the other hand, HL7 ActRelationships are more appropriate for making associations between multiple distinct observations or procedures. However, this boundary is fuzzy and there are many situations in which either approach may have equal merit.

The use of SNOMED CT attributes may result in arbitrarily complex statements that wrap multiple distinct findings within a single terminological expression. In these cases, the use of separate coded statements linked by Act Relationships is preferable. On the other hand, use of multiple statements linked by ActRelationships to represent a single composite finding or procedure may result in loss of the natural clinical term used by a clinician within a collection or linked classes.

Even when the guidelines above are followed, there will be grey areas. In an ideal world rule would be devised to compute equivalence between single Act class instances containing a post-coordinated SNOMED CT expressions and multiple Act class instances. While this is theoretically possible, there are several practical obstacles. The HL7 vocabulary for the ActRelationship.typeCode attribute differs from the range of values for linkage attributes in SNOMED CT. Simple precise or close mappings exist for some values but more work is needed before we can assert full semantic interoperability between the two representations. In addition, while a single instance post-coordinated representation has a single life-history the individual instances in multiple class representation may have separate life histories and separate associations with other contextual information.

The SNOMED CT [ 408732007 | subject relationship context ] attribute provides a mechanism for indicating that the subject of a procedure or finding is a person (or other entity) related to the subject of the record. This facility is used to define some SNOMED CT concepts (e.g. "family history of asthma" has [ 408732007 | subject relationship context ] = [ 303071001 | person in the family ]). The same attribute can also be used to create post-coordinated expressions. For example, it can be used to express a family history of a disorder without requiring the a concept that expresses a family history of that particular disorder. Unlike the HL7 "subject" participation, the SNOMED CT mechanism does not directly support reference to an identified person.

The following rules are specified to encourage explicit recording of the [ 408732007 | subject relationship context ] in order to minimize risks of overlooking this aspect of the information.

1. When using SNOMED CT to represent an observation or procedure that applies to a subject other than the record target, the appropriate [ 408732007 | subject relationship context ] SHOULD be specified in the SNOMED CT expression.
   • For example "family history" should be represented using an expression that specifies the [ 408732007 | subject relationship context ] as [ 303071001 | person in the family ].
2. The HL7 subject participation MAY also be used and SHALL be used if there is a requirement to specifically identify an individual subject.
3. If the HL7 subject participation is used in addition to the SNOMED CT representation of [ 408732007 | subject relationship context ], the Role.code of the role that is the target of the subject SHOULD be represented using SNOMED CT with the value applied to the [ 408732007 | subject relationship context ] or with a subtype of that value.

These recommendations leave some situations in which either approach may be used. Therefore, to compute equivalence, a map between the values used in the code attribute of the associated subject role is required. The simplest option would be to specify that when the classCode attribute of the HL7 Role specifies "personal relationship" the code attribute should have a value from the SNOMED CT [ 408732007 | subject relationship context ] hierarchy.

Ambiguity may be introduced if the information is coded using a concept with explicit [ 408732007 | subject relationship context ] and also has an association to a specific subject. For example, if the concept [ 160303001 | FH: diabetes mellitus ] is stated in an observation linked to a person other than the subject of the record, this could mean either (a) "the patient has a family history of diabetes, in the named family member" or (b) "the identified subject has a family history of diabetes".

Specific recommendations on this should be included in communication specifications. Where a communication pertains to an individual patient interpretation (a) is recommended. However, specific instances of the subject participation in a communication about a group of patients may need to specify interpretation (b).

Some SNOMED CT [ <<386053000 | evaluation procedure ] and [ <<363787002 | observable entity ] concepts indicate the type of specimen that is the subject of the measurement or observed property. Refinement is possible using the [ 116686009 | has specimen ] attribute to specify particular specimen types for any relevant procedure. Therefore, there is a potential overlap between two approaches to representation of the nature of the specimen.

1. When using SNOMED CT to record an observation that applies to a specimen the nature of the specimen MAY be expressed separately using the Entity.code of the entity playing the role that is the target of the specimen participation
   • This type of representation may be appropriate in cases where it is also necessary to identify the specimen and where a single specimen is the subject of multiple different observations.
   • When using this form of representation:
     • The type of specimen SHOULD be expressed using an appropriate SNOMED CT concept in the Entity.code attribute.
     • If the SNOMED CT expression used in Observation.code specifies a value for the [ 116686009 | has specimen ] attribute, the value of this attribute SHALL be either the same as or less specific than the value used in the Entity.code.
2. Alternatively, a specific SNOMED CT concept or expression MAY be used to specify the nature of the observation including the type of specimen.
   • This form MAY be appropriate to simple recording of result in a clinical record but does not allow the specific instance of the specimen to be identified.

The recommendations on representation of specimen take account of the current incomplete set of investigation codes available. Recent experience in the UK suggests that the first approach above, using the Entity.code is a more flexible basis for requesting and reporting laboratory investigation using SNOMED CT.

The guidance on use of SNOMED CT in the Entity.code attribute is intended to avoid conflicts or ambiguity that may result from representing the values of [ 116686009 | has specimen ] and Entity.code using different coding systems.

In SNOMED CT concepts that are subtypes of [ 432102000 | administration of therapeutic substance] can also specify the nature of the substance administered. Refinement of any particular type of administration is possible by applying values to the "direct substance" attribute to represent administration of any pharmaceutical product. Therefore, there is a potential overlap between two approaches to representation of the nature of the substance administered.

1. When using SNOMED CT to communicate about the supply or administration of a substance the nature of the substance SHOULD be specified in the Entity.code of the Entity associated with the Role that is the target of the relevant product or consumable Participation. When using this form of representation:
   • the Act.code of the SubstanceAdministration class instance SHOULD be coded using a SNOMED CT concept that is a subtype of [ <<432102000 | administration of therapeutic substance ], but which does not specify a [ 363701004 | direct substance ].
   • the nature of the substance administered SHOULD be expressed using an appropriate SNOMED CT concept in the Entity.code attribute of Entity playing the Role that is the target of the relevant participation.

     Example: SubstanceAdministration.code= [ 36673005 | intradermal injection ] with associated Entity (via a consumable Participation and an appropriate Role), in which Entity.code=[ <<82573000 | lidocaine ] OR

2. When using SNOMED CT to summarize information about a particular type of medication (e.g. use of a local anesthetic during a procedure), a SNOMED CT expression that includes information about the nature of the substance administered MAY be used. However, this form SHOULD NOT be used for communicating about the prescription, supply or personal administration of medication.

   Example: SubstanceAdministration.code=[ 36673005 | intradermal injection | 363701004 | direct substance | << 82573000 | lidocaine ]

The first approach follows the form recommended by the Pharmacy TC and endorsed by the Clinical Statement pattern and other domain committees. The alternative approach may be relevant for summary notes related to certain types of treatment but is not appropriate for prescribing or medication management as it does not provide a reference to a specific quantifiable amount of the substances administered nor does it allow reference to batch numbers and detailed product information.

Propagation of context is valuable and in some cases almost essential, as it reduces the need to duplicate contextual information. However, it is not entirely clear whether and if so how this propagation of context applies to coded information in each Act instance. Safe interpretation of clinical information requires a common understanding of where contextual information represented using SNOMED CT if either Act.code or Observation.value propagates to related Acts based on the context propagation rules. For example, several Observations coded using SNOMED CT disorder concepts might be related as component parts of an Organizer labeled with the SNOMED CT code "family history of disorder". If the coded context propagated it might seem to express a family history, if not these might be part of the personal medical history of the subject of record.

The following rules are specified to minimize the risk of ambiguity due to loss of contextual information.

1. SNOMED CT contextual information SHOULD NOT be assumed to propagate between Acts and SHOULD therefore be restated in each expression.
   • For example, each SNOMED CT expression in a collection of statements representing family history, SHOULD represent the relevant [ 408732007 | subject relationship context ]. This context SHOULD NOT be assumed to propagate from an Organizer (or other containing Act) to its constituent Observations or from one Observation to another.
2. In specific cases where there is clear advantage is allowing specific aspects of SNOMED CT context to conduct, this behavior SHALL be explicitly documented in a manner that ensures reproducible interpretation.

It is not clear how context conduction is intended to apply to contextual information that is represented in concepts within an Act. If this type of context is assumed to propagate it would mean that the meaning of a single Observation might be fundamentally altered by a related Act (or potentially by a chain of several different related Acts). This type of dependency presents significant risks, since different systems may be unable to reproducibly determine the composite meaning. Therefore, it seems safest to recommend restatement of the essential aspects of context as defined by the SNOMED CT context model rather than permitting this context to conduct.

There may be some specific cases, where a tightly coupled set of Acts are expected to behave as a block with regard to the surrounding context and where some or all aspects of context represented using SNOMED CT also need to be conducted. In these cases the potential for misinterpretation needs to be considered and appropriately documented.

Based on these guiding principles come the following acceptable patterns:

PATTERN ONE: Observation.code [ (<<363787002 | Observable entity) OR (<<386053000 | Evaluation procedure) ] ; Observation.value = not null (e.g. numeric, nominal, ordinal, coded result).

NOTE: At the time of this writing, the SNOMED Standards Development Organization is debating whether or not Observable entity concepts should be recommended for use in Observation.code.

<observation classCode="OBS" moodCode="EVN">
  <code code="50373000|Height|" codeSystem="2.16.840.1.113883.6.96">
    <displayName value="Height"/>
  </code>
  <text>Height: 177 cm</text>
  <value xsi:type="PQ" value="1.77" unit="m"/>
</observation>

<observation classCode="OBS" moodCode="EVN">
  <code code="247030006|Eye color|" codeSystem="2.16.840.1.113883.6.96">
    <displayName value="Eye color"/>
  </code>
  <text>Green eyes</text>
  <value xsi:type="CD" code="371246006|Green|" codeSystem="2.16.840.1.113883.6.96">
    <displayName value="Green"/>
  </value>
</observation>	

"2.16.840.1.113883.6.96" is the code system designation for SNOMED CT.

PATTERN TWO: Observation.code = "ASSERTION" (codeSystem="2.16.840.1.113883.5.4"); Observation.value [ (<<413350009 | Finding with explicit context) OR (<<404684003 | Clinical finding) ] .

<observation classCode="OBS" moodCode="EVN">
  <code code="ASSERTION" 
    codeSystem="2.16.840.1.113883.5.4"/>
  <text>Headache</text>
  <value xsi:type="CD" code="25064002|Headache|" codeSystem="2.16.840.1.113883.6.96">
    <displayName value="Headache"/>
  </value>
</observation>	

In this example, the observation is simply the assertion of a "headache". If there is a need to distinguish between, say, a patient-reported symptom vs. a clinician-asserted diagnosis, more information would need to be present. Thus, while an acceptable pattern is to assert a clinical finding, that may not convey sufficient context for all communication use cases. Likewise, an assertion of a procedure.code (such as for an appendectomy performed 5 years ago) doesn't distinguish between a patient's reported past surgical history vs. information gleaned from chart review, and additional contextual information will be needed in some cases.

<observation classCode="OBS" moodCode="EVN">
  <code code="ASSERTION" codeSystem="2.16.840.1.113883.5.4"/>
  <text>Presence of headache</text>
  <value xsi:type="CD" code="373573001|Clinical finding present|:246090004|Associated finding|=25064002|Headache|" codeSystem="2.16.840.1.113883.6.96"/>
</observation>	

In this example, a finding with explicit context is used to assert the presence of a headache.

Patterns for the common sources listed above include:

PATTERN ONE: Source is previously recorded information.

<observation classCode="OBS" moodCode="EVN">
  <id root="3568dbe1-8f49-11da-a72b-0800200c9a66"/>
  <code code="ASSERTION" codeSystem="2.16.840.1.113883.5.4"/>
  <text>Headache, per problem list</text>
  <value xsi:type="CD" code=" 25064002|Headache|"  codeSystem="2.16.840.1.113883.6.96">
    <displayName value="Headache"/>
  </value>
  <actRelationship typeCode="XCRPT" 
    contextConductionInd="false">
    <actReference classCode="OBS" moodCode="EVN">
      <id root="201877f1-8f49-11da-a72b-0800200c9a66"/>
    </actReference>
  </actRelationship>
</observation>	

This pattern uses an actRelationshipType of "XCRPT" to indicate that there is a new observation which represents an excerpt of previously recorded information. The ActReference class is used here as the target, but other clinical statement act choices could also be used. Context conduction to the ActReference class is blocked by setting contextConductionInd to "false".

PATTERN TWO: Source is informant.

The distinction between the excerpt relationship in the prior figure and an informant participant discussed here can be blurry, such as when a clinician is drawing upon the patient's recollection and a prior record of medication use to determine the current medication usage. An informant (or source of information) is a person who provides relevant information, whereas an excerpt is a sub portion of some other act.

<observation classCode="OBS" moodCode="EVN">
  <code code="ASSERTION" codeSystem="2.16.840.1.113883.5.4"/>
  <text>Father says that the patient has a headache.</text>
  <value xsi:type="CD" code="25064002|Headache|" codeSystem="2.16.840.1.113883.6.96">
    <displayName value="Headache"/>
  </value>
  <informant typeCode="INF">
    <relatedEntity classCode="PRS">
      <code code="66839005|Father|" codeSystem="2.16.840.1.113883.6.96">
        <displayName value="Father"/>
      </code>
    </relatedEntity>
  </informant>
</observation>

<observation classCode="OBS" moodCode="EVN">
  <code code="ASSERTION" codeSystem="2.16.840.1.113883.5.4"/>
  <text>Father says that the patient has a headache.</text>
  <value xsi:type="CD" code="25064002|Headache|:419066007|Finding informer|=66839005|Father|" codeSystem="2.16.840.1.113883.6.96"/>
</observation>

The first example uses an Informant participant to indicate that the observation is gleaned through the record subject's father, and the second example expresses the same thing using the finding informer attribute in a post-coordinated expression.

The first example is particularly useful where there is a need to identify or provide additional specifics about the informant participant. Where both informant participant and finding informer are present, the former should be the same as or a specialization of the latter.

<observation classCode="OBS" moodCode="EVN">
  <code code="ASSERTION" codeSystem="2.16.840.1.113883.5.4"/>
  <text>Patient states he has a headache</text>
  <value xsi:type="CD" code="25064002|Heachache|:419066007|Finding informer|=116154003|Patient|" codeSystem="2.16.840.1.113883.6.96"/>
</observation>	

This example shows the use of the finding informer attribute to indicate that the patient is the source of the information. It will commonly be the case that a V3 instance will assert an informant participant, which will propagate to nested observations. Therefore it won't often be necessary to directly assert a finding informer of patient.

PATTERN THREE: Source is direct examination.

<observation classCode="OBS" moodCode="EVN">
  <code code="77989009|Measurement of skin fold thickness|:370129005|Measurement method|=5880005|Physical exam|" codeSystem="2.16.840.1.113883.6.96"/>
  <text>Skin fold thickness is 7cm</text>
  <value xsi:type="PQ" value="7" unit="cm"/>
</observation>

This pattern uses the SNOMED CT measurement method attribute to qualify a measurement procedure concept, indicating that the observation was determined via physical exam.

<observation classCode="OBS" moodCode="EVN">
  <code code="ASSERTION" codeSystem="2.16.840.1.113883.5.4"/>  
  <text>Hilar mass on chest CT</text>
  <value xsi:type="CD" code="309530007|Hilar mass|:418775008|Finding method|=169069000|CT chest|" codeSystem="2.16.840.1.113883.6.96"/>
  <actRelationship typeCode="SUBJ" contextConductionInd="false">
    <observation classCode="DGIMG" moodCode="EVN">
    <id root="9cc8b460-8f47-11da-a72b-0800200c9a66"/>
      <code code="169069000|CT chest|" codeSystem="2.16.840.1.113883.6.96">
        <displayName value="CT chest"/>
      </code>
    </observation>
  </actRelationship>
</observation>	

This pattern uses the SNOMED CT finding method attribute to qualify a finding concept, indicating that the finding was determined via CT chest. To relate the finding to the actual CT scan being observed, the example uses an act relationship of type "SUBJ", with blocked context conduction.

A large clinical terminology, such as SNOMED CT, represents a number of lexically similar concepts which are grammatically, linguistically or semantically distinct. This phenomenon is particularly pronounced if the terminology is considered without any kind of partitioning. The coarse-grained partitioning specified by these constraints simplifies and clarifies decisions about which of a set of superficially similar SNOMED CT concepts are appropriate to particular HL7 vocabulary domains.

For example, consider a vocabulary domain specification that is intended to represent "an adverse event in reaction to a drug".

• The most suitable SNOMED CT concepts to represent such an event would be those subsumed by [ <<62014003 | adverse reaction to drug ].
• However from a lexical perspective other less appropriate concepts may appear to be suitable. For example
  • The reference to "adverse drugs reaction" may suggest use of subtypes of the procedure concept [ <<396079007 | assessment of adverse drug reactions ].
  • The reference to "drug" may suggest concepts in the use of subtypes of [ <<373873005 | pharmaceutical / biologic product ] or [ <<410942007 | drug or medicament ].

The Schematic Illustrations of SNOMED CT Expressions (§ E.6 ) identify the "clinical kernel" or primary clinical "focus concept" that may exist alone or as part of a contextualized expression. In most cases, the simple value set constraints in this specification apply to this clinical focus concept. In combination with the SNOMED CT concept model these constraints form a foundation for more detailed "complete" value set specifications (as explored in Detailed aspects of issues with a vocabulary specification formalism (§ E )).

Simple value set constraints can be regarded as a set of subsumption clauses related by OR logic. Each clause permits the inclusion of a focus concept that is subsumed by a specified concept. In contrast, a more complete specification would check normal form transformations of candidate expressions against a variety of subsumption and role-based restrictions. In addition a complete specification require support of a full set of logical operators between clauses (i.e. OR, AND, NOT).

For example, consider a value-set constraint which indicates that the "focus concept" must be a kind of [ <<404684003 | clinical finding ].

• The concept model indicates that a [ 404684003 | clinical finding ] concept
  • can be refined by name/value pairs with attribute names such as [ 363698007 | finding site ], [ 246112005 | severity ], [ 116676008 | associated morphology ] etc.,
  • can be the value to the attribute name [ 246090004 | associated finding ]
    • as part of the definition or refinement of a [ 413350009 | finding with explicit context ]
    • as part of post-coordinated expression that includes the [ 404684003 | clinical finding ] within a context wrapper.

• A comprehensive notation for value sets that allow subtypes of [ 404684003 | clinical finding ] may therefore also need to indicate whether any limitations apply either to the refinement or situation in which the concepts are used.
  • The context wrapper may, for instance, be used to communicate negation and uncertainty and may thus be required to support some types of information. However, it may also be necessary to constrain the use of context in a manner that is relevant to the Act.moodCode or other attributes and association in the HL7 representation.

Two different aspects of SNOMED CT post-coordination ("attribute refinement" and "context/situation wrapping") may result in the valid expressions being rejected by "simple" value sets tests.

Example of "attribute refinement" false negative:

The concept [ 82764005 | operation on duodenum ] could be refined by applying more specific values to its [ 260686004 | method ] and [ 363704007 | procedure site ] attributes.

If the value for [ 260686004 | method ] is refined to [ 129304002 | excision - action ] and the value for [ 363704007 | procedure site ] to [ 181247007 | entire duodenum ], the resulting expression means "excision of the entire duodenum" and we would expect this to mean the same as "total excision of duodenum". This expression would be inappropriately rejected by a "simple" value set test of the instruction [ <<173848007 | total excision of duodenum ] (i.e. "include 'total excision of duodenum' any sub-types").

• This false negative arises because [ 82764005 | operation on duodenum ] is not a subtype of [ 173848007 | total excision of duodenum ].
• In order to obtain the appropriate result, a more complex test must be performed. This involves comparison of the normal forms of the two expressions.

Example of "context/situation wrapping" false negative:

The concept [ 373573001 | clinical finding present ] can be refined by applying a more specific value to its attribute [ 246090004 | associated finding ].

If the value for [ 246090004 | associated finding ] is refined to [ 404640003 | dizziness ], the resulting post-coordinated expression means "dizziness present". This expression would be inappropriately rejected by a "simple" value set test of the instruction [ <<404640003 | dizziness ] (i.e. "include 'dizziness' and any sub-types").

• This false negative outcome is because [ 373573001 | clinical finding present ] is not a subtype of [ 404640003 | dizziness ].
• In order to obtain the appropriate result, a more complex test must be performed. This involves comparison of the normal forms of the two expressions, taking account of the default context of a SNOMED CT 'finding'.

A potential pattern of false positive value set testing would result from attempts to augment the "simple" value set specifications to include corresponding "context/situation" Concepts. The absence (by design) of an exhaustive set of "context/situation" Concepts corresponding to each "finding" or "procedure" Concept means that on many occasions only the specification of a more general "situation" Concept will guarantee that desirable Concepts will be included, but at the expense of allowing multiple false positives.

Example of pre-coordinated "situation" false positive:

Consider a value set designed to include "respiratory disorders". To avoid rejecting concepts which include explicit context, a simple value set might include [ <<413350009 | finding with explicit context ] as well as [ <<50043002 | disorder of respiratory system ]. Such a clause would include the relevant respiratory findings, including those with explicit context. However, it would also inappropriately include other findings with explicit context (i.e. non-respiratory finding with explicit context).

Failure to include [ <<413350009 | finding with explicit context ] would result in false negatives as illustrated in the previous section.

• In order to obtain the appropriate result, a more complex test must be performed. This involves restricting the inclusion of subtypes of [ 413350009 | finding with explicit context ] to those with a value for [ 246090004 | associated finding ] that are in the set specified by [ <<50043002 | disorder of respiratory system ].

A further alternative representation is needed to communicate record entries where SNOMED CT content has been used to represent Observation.code and Observation.value is present. Observation.value may be a numeric, nominal or ordinal result, so itself may come from SNOMED CT also:

The following very general SNOMED CT value set for using the Entity.code attribute is outlined below. In any specific model this set should be appropriately constrained.

A comprehensive notation for all SNOMED CT ‘findings and procedures" value sets is logically ‘wrapped" in the SNOMED CT context/situation wrapper, and indeed the context/situation wrapper would be used to communicate negation and uncertainty in message designs where SNOMED CT is the only permitted code system. In more "complete" value set constraint specifications therefore, it can be expected that the moodCode values found in message instances should influence the corresponding valid "finding and procedure context" values. Details of the recommended mappings are provided in Act.moodCode (§ 2.2.3 )

A value set constraint can be applied to any coded content where the codeSystem is SNOMED CT. This includes cases where original encoding is SNOMED CT or where the SNOMED CT encoding is based on a translation from another codeSystem. Thus the value set constraints may be tested against the original encoding or translation sub-element in an instance of the HL7 Concept Description (CD) data type.

New record entries should be made using SNOMED CT concepts with an active status. However it is possible that communications may contain SNOMED CT content that, while active at the time of entry, have subsequently been rendered inactive in the reference data(e.g. as a result of recognition or errors such as duplication or ambiguity). In these cases value set testing SHOULD include analysis of information contained in the SNOMED CT history data. Such data will assist in establishing the relationship(s) between inactive concepts and active concepts. If it can be demonstrated that an inactive concept has an appropriate historical relationship to a value set valid active concept, and if the specification does not explicitly exclude inactive concepts, then the inactive concept should be regarded as valid for communication.

For example, consider the concept [ 274638001 | asthenia ], which is now marked as an inactive duplicate in SNOMED CT. This concept may have been active in the past, and may thus have been used in the creation a record entry. This historical record entry may subsequently be communicated (perhaps as part of a record extract), by which time the concept has been marked as inactive. If this is encountered it is possible (by analysis of the SNOMED CT history data) to identify the [ 168666000 | SAME AS ] relationship to the active concept [ 13791008 | asthenia ]. Assuming the message specification does not explicitly exclude inactive concepts it is then possible to test the (active) concept for suitability in the message instance and accept it as valid.

Where communicating parties agree that only ConceptId’s are required for communication, whether single Id’s or compositional code phrases, these SHALL be communicated using CD.code, with expressions structured according to the SCG.

<value code="66308002" codeSystem="2.16.840.1.113883.6.96"/>

<value code="127278005:363698007=85050009,116676008=72704001" codeSystem="2.16.840.1.113883.6.96"/>

It is, however, likely that good recording/communication practice between communicating parties will require the communication of associated human readable elements. Guidance is therefore provided for the following circumstances:

Where a Term of a valid Description for the communicated SNOMED CT ConceptId has been selected to make the originating record entry, or where communicating parties wish to communicate a valid Description for a code it may be communicated as:

CD.displayName - subject to the rules of the terminology, e.g. by use of a designated reference set that specifies the term to be selected for each code:

<value code="66308002" codeSystem="2.16.840.1.113883.6.96">
  <displayName value="fracture of humerus"/>
</value>

CD.code, using the SCG rules:

<value code="66308002|fracture of humerus|" codeSystem="2.16.840.1.113883.6.96"/>

Both CD.code and CD.displayName:

<value code="66308002|fracture of humerus|" codeSystem="2.16.840.1.113883.6.96">
  <displayName value="fracture of humerus"/>
</value>

Where both CD.code and CD.displayName are used, the terms must be the same.

CD.originalText may, of course, also be communicated - subject to the rules of the data type specification:

<value code="66308002" codeSystem="2.16.840.1.113883.6.96">
  <displayName value="fracture of humerus"/>
  <originalText mediaType="text/plain" value="fracture of the humerus"/>
</value>

Where either a pre-crafted human-readable string or a relevant fragment from analysed narrative text is associated with a single code or compositional SNOMED CT Expression, this string SHALL be communicated as CD.originalText:

<code code="387713003:363704007=264116001,260507000=129236007,260686004=257903006" codeSystem="2.16.840.1.113883.6.96">
  <originalText mediaType="text/plain" value="Open repair of outlet of muscular interventricular septum"/>
</code>

Where the recording process also presents a valid SNOMED CT Description (or Descriptions) to assist in the selection/creation of the communicated SNOMED CT Expression, or where communicating parties wish to communicate a valid Description for a code (or each code in a compositional expression) the associated Term (or set of Terms) MAY be communicated as follows:

<code code="387713003|Surgical procedure|:363704007|Procedure site|=264116001|Outlet muscular septum|,260507000|Access|=129236007|Open approach - access|" 
      codeSystem="2.16.840.1.113883.6.96">
  <originalText mediaType="text/plain" value="Open repair of outlet of muscular interventricular septum"/>
</code>

If communicating parties agree that CD.code will only convey ConceptIds, then there is no current support, according to the rules of the datatype specification and the SCG rules, for unambiguously communicating Descriptions using available CD attributes.

In future, if alternative standard term-phrase composition rules become part of the SNOMED CT standard (and regarded as such by relevant communicating parties) then the value of displayName could be generated according to these rules.

If neither a pre-crafted human-readable string, nor a relevant fragment from analysed narrative text is associated with a single code or compositional SNOMED CT Expression, then:

1. The minimal representation pattern MAY be used (if this is regarded as satisfactory for recording/communication purposes between communicating parties) – see ‘Minimal representation’ above.
2. If recording process also presents a valid SNOMED CT Description (or Descriptions) to assist in the selection/creation of the communicated SNOMED CT Expression, the associated Term (or set of Terms) must be communicated in CD.code, structured according to the SCG rules (note, this is the convention used in examples elsewhere in this implementation guide):

<code code="387713003|Surgical procedure|:363704007|Procedure site|=264116001|Outlet muscular septum|,260507000|Access|=129236007|Open approach - access|" 
      codeSystem="2.16.840.1.113883.6.96"/>