Product GELLO

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Product Brief - GELLO

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Product Name

  • HL7 Version 3 Standard: Gello: A Common Expression Language, Release 2
  • GELLO: An OCL-Like Common Expression Language, Release 2


Standard Category

Health Information Knowledge Representation Standards

Integration Paradigm


Normative, ANSI Standard


  • ANSI/HL7 V3 GELLO, R1-2005; R2-2010
  • HL7 GELLO, R2 (2009)


GELLO is a standard query and expression language for decision support. The syntax of the GELLO language can be used with any object-oriented data model, and is based on the Object Constraint Language (OCL). OCL was developed by the Object Management Group (OMG) as a constraint and query language for UML class models. Given that the HL7 Version 3 Reference Information Model (RIM) and associated Refined Message Information Models (R-MIMs) are based on UML, GELLO was designed to leverage the semantics of these HL7 models, in combination with HL7 Vocabulary and Data Types, for clinical decision support.


HL7 Version 3 Standard: GELLO, A Common Expression Language is intended to be a standard query and expression language for decision support. GELLO is a class-based, object-oriented (OO) language that is built on existing standards. GELLO includes both query and expression sublanguages, which we refer to collectively as the GELLO language (for succinctness we will also refer to the two sublanguages simply as languages). GELLO is based on the Object Constraint Language (OCL), developed by the Object Management Group. Relevant components of OCL have been selected and integrated into the GELLO query and expression languages to provide a suitable framework for manipulation of clinical data for decision support in health care. The GELLO language can be used to:

  • Build up queries to extract and manipulate data from medical records.
  • Construct decision criteria by building up expressions to reason about particular data features/values. These criteria can be used in decision-support knowledge bases such as those designed to provide alerts and reminders, guidelines, or other decision rules.
  • Create expressions, formulae, and queries for other applications.

Business Case (Intended Use, Customers)

  • Clinicians,
  • researchers,
  • epidemiologists,
  • knowledge engineers,
  • programmers,
  • health economists


A developer/user of computerized clinical guidelines and clinical knowledge has come across valuable pieces of encoded clinical knowledge that s/he wishes could be incorporated in his/her systems. However, the proliferation of multiple, incompatible clinical applications, each based on its own hardware architecture, platform, and operating system; each having patient records stored in ad hoc databases specifically tailored to satisfy the needs of a particular organization, 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.
There is a better way for doing this: with GELLO, clinical knowledge (e.g. decision rules, eligibility criteria, patient state specifications) can be represented with expressions that are:

  • Specifically targeted to clinical applications
  • Object oriented
  • Platform independent
  • Vendor independent
  • Intended to access data through an OO data model – an RMIM compatible with the HL7 RIM
  • Robust, strongly-typed, declarative and side-effect free.

Thus use of GELLO:

  1. provides platform-independent support for mapping to the OO data model used, therefore eliminating the need for implementation-specific encoding methods for information retrieval as part of knowledge content (guidelines, alerts, etc.).
  2. simplifies the creation and updating of clinical data objects, and their evaluation.
  3. facilitates sharing of decision logic and other computer expressions.
  4. reduces errors and discrepancies in knowledge representation


Work Groups


Relationship to/ Dependencies on, other standards

  • OCL (by OMG)
  • RIM
  • In regard to existing standards for decision support, the evolution of Arden Syntax data references to compatibility with GELLO is already under way. GELLO is not intended to be a substitute for Arden Syntax, although it could be used to write MLMs. In addition, however, GELLO can be used to support a variety of specifications of expressions in decision support settings other than MLMs.

Links to current projects in development

  • Project Insight ID #369 GELLO v.1 Revision and Implementation Guide
  • Project Insight ID #229 HL7 GELLO v.2 Syntax Re-Ballot