Section 3: Clinical and Administrative Domains
Section 6: Rules and References

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GELLO (HL7 Version 3 Standard: Gello: A Common Expression Language, Release 2)


GELLO, part of the HL7 Version 3 Normative Edition, is a standard query and expression language that provides a suitable framework for manipulation of clinical data for decision support in healthcare. The GELLO language can be used to build queries to extract and manipulate data from medical records and construct decision criteria by building expressions to correlate particular data properties and values. These properties and values can then be used in decision-support knowledge bases that are designed to provide alerts and reminders, guidelines, or other decision rules.


GELLO (HL7 Version 3 Standard: Gello: A Common Expression Language, Release 2) may also go by the following names or acronyms:



  • Clinicians
  • Researchers
  • Epidemiologists
  • Knowledge Engineers
  • Programmers
  • Health Economists


  • Facilitates sharing of decision logic and other computer expressions
  • Reduces errors and discrepancies in knowledge representation
  • Simplifies the creation and updating of clinical data objects and their evaluation
  • Provides platform-independent support for mapping to the Orders & Observations data model used, therefore eliminating the need for implementation-specific encoding methods for information retrieval as part of knowledge content (guidelines, alerts, etc.)


See the Implementation Guide for this Standard.

  • Numerous implementations, including Infermed in the United Kingdom and Medical Objects in Australia. Case studies are in development


GELLO is a class-based, object-oriented (OO) language that is based on the Object Constraint Language (OCL), developed by the Object Management Group (OMG). Relevant components of OCL have been selected and integrated into the GELLO query and expression languages. A developer/user of computerized clinical guidelines and clinical knowledge has valuable pieces of encoded clinical knowledge that he/she wishes to incorporate in his/her systems. However, the proliferation of multiple, incompatible clinical applications precludes the reuse of valuable knowledge and incorporating it in clinical applications without the painful process of restructuring and sometimes, rewriting it all to meet particular specifications. GELLO resolves this with clinical knowledge (e.g. decision rules, eligibility criteria and patient state specifications) that can be represented with expressions that are specifically targeted to clinical applications; object oriented; platform and vendor independent; robust, strongly typed, declarative and side effect free.


Clinical Decision Support


GELLO (HL7 Version 3 Standard: Gello: A Common Expression Language, Release 2)

(Download) (375 KB)

HL7 Version 3 Standard: Gello: A Common Expression Language, Release 1 (View Brief)

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HL7 Version 3 Implementation Guide: Virtual Medical Record for Clinical Decision Support (vMR-CDS) for GELLO, Release 1 (View Brief)


The standard is also available as part of the Health Level Seven Version 3 (V3) Normative Edition. See the HL7 Version 3 Product Suite for downloads.