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Policy on OIDs for SNOMED CT Extensions

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There has been a request for several new terms to support the imminent guidelines for Newborn Screening that will be content for the new US Extension to SNOMED CT. The question has arisen as to what OID to use? Should extensions have their own OIDs or not? How will this affect translation and mapping? This was discussed at length on the March 17, 2011 Vocabulary WorkGroup conference call, and the discussion and decisions reached by the workgroup are summarized here.


Current vocabulary guidance implies that a particular code system will be all-inclusive for the terminology included in that code system. The problem is if the SNOMED OID is used for an extension, the extension content is not in SNOMED International, and it is tedious for someone to find information about a particular code.

The issue is whether HL7 needs to provide some kind of traceability process so users can find source information about a particular extension code. One solution to solve such problems would be a different OID registered to any SNOMED CT extension (not just national extensions) with the traceability and provenance information in the description of the new OID. Beverly related that when this approach was taken for the LOINC extensions defined in Canada, implementation issues arose as implementers find it very inconvenient and unpleasant to have to use different OIDs for LOINC core and the extension. The solution of a single combined/integrated release in Canada is that other extensions and new LOINC releases could have code and term collisions. After much discussion, it was realized that different approaches may be taken depending upon how the extensions to a terminology are managed by the author/publisher of the terminology. In the case of the emerging policies at IHTSDO, management of the concept identifiers for extensions will ensure that new ones added independently by different nations or other extension bodies will not collide. However, this was done very deliberately within the technical framework for namespace and concept identifier managment by IHTSDO, the Member countries and Affiliate organizations. It must be kept in mind that the base assumption for everyone should be that every value set that you use could potentially have codes from more than one code system.

Proposed Policy

The following policy was articulated by the HL7 Vocabulary WorkGroup:

  • If a terminology does not manage/use/provide for any extensibility in the code system, then a different OID is needed for any extension codes in any separate namespace (e.g. ISO Country Codes).
  • If a terminology has extension mechanisms, but does not manage the extension namespaces such that it would be possible for extenders to define the same codes independently with different meanings (e.g. LOINC), a different OID for each extension must be defined.
  • If a terminology manages all namespaces correctly so as to avoid any collisions or ambiguities (e.g. SNOMED CT), then a different OID is not needed.

This policy was approved unanimously on a vote on the conference call.