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Announcing the retirement of SNOMED CT UK Drug Extension

This is another follow-up on SNOMED overhaul announcement.

Currently, SNOMED Vocabulary is ingested into OMOP vocabularies as an amalgamation of multiple sources:

  • SNOMED CT International,
  • SNOMED CT US local edition
  • SNOMED CT UK local edition.

The latter in turn is built from two modules: Clinical and Drug extension.

The decision has been made to exclude all existing SNOMED CT UK Drug Extension content, and to stop supporting it in the future. The reasoning is the following:

  • dm+d exists. It is maintained by the same subdivision within the UK NHS, the content between SNOMED UK Drug extension and dm+d is the same, they intentionally use the same SCT IDs as codes for concepts, and although concepts chronologically are introduced to SNOMED first, they find their way to dm+d within one month in 96% of all investigated cases.
  • NHS is radically changing editorial policy, which is caused by misalignment with SNOMED International’s plans for changes in drug product hierarchy. That means UK Drug Extension will keep intentionally introducing competing hierarchies and duplicate codes for existing content. This is not a problem for dm+d, but poses new challenges for unified SNOMED Vocabulary in OMOP CDM, which has always relied on RF2 sources to fix duplication, not introducing it as a part of the design.
  • We observe UK Drug Extension reactivating concepts, retired by SNOMED International. This is discouraged by shared SNOMED guidelines, and breaks our established assumption of SNOMED core module primacy, threatening even more lasting discrepancies among merged national modules. Currently, only a select few non-standard concepts are affected, but each recent release continuously introduced more, indicating editorial policy change.
  • Absence of Standard content. Currently, only Device and Route concept domains have Standard concepts that are usable inside OMOP CDM. Devices are full duplicates of dm+d concepts, which we will turn Standard in dm+d, and Routes are unmaintained: duplicates, known since 2019 are still Standard, and would still be. The main reason we are unconcerned about removing currently Standard Routes is the fact that the new Route hierarchy is also in work for this release.

We consider this a purely technical change, to simplify existing Standard Vocabulary by removing full duplicate concepts of dm+d and to align it with the priorities defined by the Committee. Drug concepts in SNOMED CT UK were never standardized to the RxNorm/RxNorm Extension hierarchy (unlike, again, dm+d concepts), and we do not see any benefit in continuously maintaining them separately.

As an implementation detail, the concepts are planned to be deprecated and made unavailable to use, like it was previously done with other vocabularies. As the SNOMED CT sources structure is modular, only SNOMED CT UK Drug Extension Module and accompanying metadata modules will be removed, without affecting other concepts. Of course, replacement relationships will be made wherever possible.

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