5.4. SNOMED Clinical Terms and HL7

HL7 develops standards for the exchange of health related data. Most new HL7 standards developed over the last few years have been based on HL7 Version 3. The key features of HL7 Version 3 include:

• A Reference Information Model (RIM):
  • This provides a framework for the structure of communicated information.
• A formal development method:
  • This described the various steps to turn a set of requirements into appropriate models and specifications that support communication of the necessary information.
• Separation between logical models and implementation technologies:
  • Many implementations use XML but other technical approaches can be applied to implement the same models.
• Use of external codes systems and terminologies to represent concepts:
  • Model specifications include coding constraints expressed in abstract terms as Concept Domains which are implemented as Value Sets drawn from one or more code systems.

Many HL7 Version 3 models use SNOMED CT and this has led to growing demand for guidance on consistent patterns of use. In 2004, the HL7 Vocabulary Technical Committee launched the TermInfo Project to address this requirement. The project was initially conceived as having two work packages:

1. Specification of a general approach to resolving issues related to the interface between HL7 information models and terminologies or code systems;
2. A guide to use of SNOMED Clinical Terms in HL7 Version 3 communication standards.

The SNOMED CT specific package was actively supported by SNOMED International as part of a charter agreement with HL7. After several rounds of revision and review, the 'Guide to Use of SNOMED CT in HL7 Version 3 ' was accepted by HL7 as a Draft Standard for Trial Use (DSTU).

The guide itself contains both normative and informative sections. The normative sections cover:

• Guidance on dealing with specific overlaps between RIM and SNOMED CT semantics and recommendations for use of SNOMED CT in relevant Attributes of various RIM classes;
• Constraints on SNOMED CT concepts applicable to relevant Attributes in each of the major classes in the Clinical Statement pattern.

The informative sections cover:

• Examples and patterns for representing common clinical statements;
• A general discussion of the potential overlaps between an information model and a terminology model and the pros and cons of various possible approaches to their management;
• References to relevant documents and known open issues.

Following adoption the TermInfo group have encouraged in-use testing and have elicited and addressed comments on the 'Guide to Use of SNOMED CT in HL7 Version 3 . Implementer feedback has further contributed to growing understanding of the issues and resulted in refinement of the guidance. Many of the recommendations included in the 'Guide to Use of SNOMED CT in HL7 Version 3 ' have been incorporated into domain specific HL7 Standards and national implementations.

One of the conclusions from the TermInfo group is a recognition that terminology and information models are co-dependent. They need to evolve collaboratively to meet requirements for unambiguous processable representations of information. Work with other Information Models (including EN13606 and openEHR) indicates that the issues raised by TermInfo are not specific to SNOMED CT and HL7 ( Representing clinical information using SNOMED Clinical Terms with different structural information models). Harmonization efforts involving the SNOMED International and HL7 and other standards bodies continue to address these issues.

As part of the ongoing harmonization work the copyright of the ' Guide to use of SNOMED CT in HL7 Version 3 ' is jointly held by HL7 and SNOMED International. In line with HL7 policies, the document expired as a formal HL7 DSTU after two years but it is included here in full as it continues to serve two Roles:

• A Description of the challenges of integrating an expressive terminology, such as SNOMED CT , with a rich information model;
• A pragmatic interim approach to these challenges, which allows for and anticipates the evolution of a more integrated solution.

[¹ There is also a precoordinated Concept "Calculus of gallbladder with cholecystitis" which is equivalent to this postcoordinated combination.

[² In practice the relationship 116680003 |is a| = 80146002 |appendectomy| is represented via intermediate supertype and is also represented by the following defining characteristics 260686004 |method|, 405813007 |procedure site - Direct| = 66754008 |appendix structure| .

[³ This is not the case in the first release as the appropriate defining characteristics are not in the release set.