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The current interpretation of Clause 4.1 with regards to creation of new concepts is that the new concepts would necessarily be leaf nodes to the existing International release. This type of content, by its very nature would have no impact on the international content, as it would be distal in the hierarchy to all international content. National extensions have found that, due to gaps in the current International release, it is often necessary to insert intermediate (i.e. non-leaf) concepts into the terminology to meet user requests as well as to provide additional navigational support. These intermediate concepts may be either primitive or fully-defined.
In general. the identification of a need for an intermediate concept in the International release indicates a gap in content coverage. Unless realm specific, requests for addition of the intermediate concept should be forward to SNOMED International for disposition by the editorial staff. It is recognized; however, that there are instances where the addition of an intermediate concept may be required initially in an extension. The primary motivation for the promotion of intermediate extension concepts is to support the needs for cross-border exchange of health data. If intermediate concepts resided solely in national extensions, then countries would need to reconcile multiple extensions in order to get a full representation of the structured data they may receive. Also, there is a potential issue with divergent definitions of similar content if it is solely maintained in an extension.
The creation of intermediate concepts necessarily results in changes to the relationships in the core terminology. These changes can be of almost any type including: assignment of new parent(s), retirement of inferred or stated parents, inheritance of new/additional defining relationships, removal of inferred relationships by the classifier, etc. There are (at least) 2 ways in which an extension concept can become an "intermediate" concept in an edition:
- The creation of a new fully defined extension concept as a subtype of an international concept. The extension concept's definition may result in it being classified as a supertype of an international concept. In this case, the transitive closure of the international edition stays the same, except for the addition of the new inferred |is a| relationships for the extension concept. The defining attribute relationships of the international subtypes stay the same; however, results in a redundant inferred |is a| relationship in the national edition.
- A new extension concept (fully defined or primitive) is created and used as the destination of a stated |is a| relationship from an international concept, meaning that the new extension concept is a supertype of an international concept. An example of this might be the creation of an "A or B" concept, where the relationships "A is a 'A or B'" and "B is a 'A or B'" are stated. In this case, the extension is changing the stated definition of an existing international concept (the subtypes), which is more likely to lead to significant consequences.
Proposed policy
SNOMED International recognizes that SNOMED CT is not always comprehensive in its terminological coverage in particular domains and that there exist "gaps" in the hierarchies, where concept coverage is not adequate for extensions. It is proposed that intermediate concepts may be added to extensions under the following guidelines:
- The addition of the concept does change the transitive closure for the International Release, except to add additional rows that include the new intermediate concept). The transitive closure of the national edition must be a superset of the transitive closure of the international edition. This necessitates the retention of all transitive IS A relationships from the core.
- No stated or inferred defining attribute of core content are inactivated in the extension.
- Inferred |is a| relationships should only be inactivated due to redundancy, but should still represent a valid relationship in the transitive closure. See "Retire (redundant) IS A relationships (not necessarily inferred)".
- No stated IS A relationships from a core concept to an extension concept is allowed.
- Additional inferred defining relationships to not create crossovers (i.e. subtypes with attribute values that are supertypes of the values represented by the same attribute of the parent)
- Additional inferred IS A relationships of subtypes do not represent a change in meaning as represented by the Fully Specified Name.
- The intermediate concepts are submitted for promotion to the international release as soon as possible.
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Full definition of intermediate concepts will in many cases result in the reassignment of core relationship due to inferences assigned from the classifier (see next section). Likewise, any subtypes that are inferred under this concept may have their inferred defintions changed (depending on the modeling pattern applied to the extension concept).
Proposed policy
- The creation of intermediate fully defined concepts within an extension is allowed, contingent on compliance with the proposed policy for creation of new concepts above.
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