HL7 Object Identifier (OID) Registry
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Introduction for the HL7 Object Identifier (OID) Registry

ISO Object Identifier (OID) Definition:

An OID is a globally unique ISO (International Organization for Standardization) identifier.� There are multiple ways that this identifier may be represented, and HL7 has chosen to represent OID registered here and used in HL7 models using a form that consists only of numbers and dots (e.g., "2.16.840.1.113883.3.1").� OIDs are paths in a tree structure, with the left-most number representing the root and the right-most number representing a leaf.

Each OID is created by a Registration Authority. Each of these authorities may, in turn, delegate assignment of new OIDs under it to other registration authorities that work under its auspices, and so on down the line. Eventually, one of these authorities assigns a unique (to it) number that corresponds to a leaf node on the tree. The leaf may represent a registration authority (in which case the OID identifies the authority), or an instance of an object. A registration authority owns the namespace consisting of its sub-tree.

OIDs are a preferred scheme for unique identifiers in HL7. OIDs should always be used except if one of the inclusion criteria for other schemes apply. HL7 Version 3 models use OIDs to identify coding schemes and identifier namespaces. OIDs can be allocated by any organization using a unique OID root. A single message can use OIDs from various sources and a single scheme can be identified by more than one OID (e.g. by an OID from more than one organization). Once issued, an OID is never withdrawn and always identifies the same scheme or object.

As with all identification systems there is not much point having identifiers if you don�t know what they represent and vice versa - hence identification systems generally require a registry. This HL7 OID registry serves that purpose for the HL7 community.

HL7-Assigned OIDs

HL7 has established this OID registry and assigns OIDs in its branch for HL7 users and vendors upon their request in its role as an ISO Registration Authority. The cost of an OID is non-refundable. HL7 also assigns OIDs to public identifier-assigning authorities both U.S. nationally (e.g., the U.S. State driver license bureaus, U.S. Social Security Administration, HIPAA Provider ID registry, etc.) and internationally (e.g., other countries Social Security Administrations, Citizen ID registries, Passport Numbers, etc.).

The HL7 registered OIDs must be used for these organizations, regardless whether these organizations have other OIDs assigned from other sources.

When assigning OIDs to third parties or entities, HL7 shall investigate whether an OID is already assigned for such entities through other sources. It this is the case, HL7 shall record such OID in a catalog, but HL7 shall not assign a duplicate OID in the HL7 branch. If possible, HL7 shall notify a third party when an OID is being assigned for that party in the HL7 branch.

Though HL7 shall exercise diligence before assigning an OID in the HL7 branch to third parties, given the lack of a global OID registry mechanism, one cannot make absolutely certain that there is no preexisting OID assignment for such third-party entity. Also, a duplicate assignment can happen in the future through another source. If such cases of supplicate assignment become known to HL7, HL7 shall make efforts to resolve this situation. For continued interoperability in the meantime, the HL7 assigned OID shall be the preferred OID used.

While most owners of an OID will "design" their namespace sub-tree in some meaningful way, there is no way to generally infer any meaning on the parts of an OID. HL7 does not standardize or require any namespace sub-structure. An OID owner, or anyone having knowledge about the logical structure of part of an OID, may still use that knowledge to infer information about the associated object; however, the techniques cannot be generalized.

Example for a tree of ISO object identifiers. HL7's OID is 2.16.840.1.113883. (link to graphic opens in a new window)

An HL7 interface must not rely on any knowledge about the substructure of an OID for which it cannot control the assignment policies.

Literal Form

Although there are three different forms that ISO defines for representing an OID, within HL7, OIDs are used as numeric UID strings only, i.e., the literal string value is the only thing that is communicated and is the only thing that a receiver should have to consider when working with UIDs in the scope of the HL7 specification. �The string is in the �dot-notation� form; see the ISO and ITU standards or the FAQ for more information.

For compatibility with the DICOM standard, the literal form of the OID should not exceed 64 characters. (see DICOM part 5, section 9).

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