SNOMED CT is a multilingual clinical terminology that covers a broad scope. However, some users may need additional concepts, relationships, descriptions or reference sets to support national, local or organizational needs. SNOMED CT is designed to allow the International Edition to be enhanced by creating extensions that meet national or local requirements. The extension mechanism allows SNOMED CT to be customized to address the terminology needs of a country or organization that are not met by the International Edition. An extension may contain components (i.e. concepts, descriptions or relationships) and/or reference sets used to represent subsets, maps or language preferences.
SNOMED CT extensions can support a variety of use cases, including:
Translating SNOMED CT, for example
Adding terms used in a local language or dialect
Adding concepts that are only relevant to a local context
Mapping between SNOMED CT and other code systems, for example
Representing maps between SNOMED CT concepts and codes from other code systems
Extending the expressivity of SNOMED CT, for example
Extending the concept model by introducing new attributes to meet specific data retrieval use cases
The logical design of a SNOMED CT extension is technically consistent with that of the International Edition. Both represent and version SNOMED CT components and reference sets in release files that conform to the Release Format 2 specification. Every SNOMED CT extension includes one or more modules, and each module contains either SNOMED CT components or reference sets (or both). A SNOMED CT extension is published as a SNOMED CT edition, which includes the contents of a focus module from the extension, together with the contents of all the modules on which it depends. This includes the modules in the International Edition and possibly other modules from a national and/or local extension. National and local extensions are managed by SNOMED International Members or Affiliates who have been issued a namespace identifier by SNOMED International. A namespace identifier is used to create globally unique SNOMED CT identifiers for each component (i.e. concept, description and relationship) and reference set within a Member or Affiliate extension. This ensures that references to extension concepts contained in health record data are unambiguous and can be clearly attributed to a specific issuing organization. Once a namespace identifier has been obtained from SNOMED International, an extension producer must create one or more module concepts, which will be used to organize the extension content. All components and reference set members in an extension must belong to a module created by the responsible organization.
The key steps in producing a SNOMED CT extension are:
It is important for extension producers to understand the complexity of authoring and managing an extension, and to prioritize the acquisition of appropriate tools to support the extension management processes. These tools are required to maintain the quality and integrity of SNOMED CT content, and to prevent errors which can easily be identified using automated processes. Extensions and editions should never be handcrafted or managed manually using a file-based approach (e.g. using spreadsheets). While a manual approach may be possible in the short term (given a solid understanding of the principles), version management for an evolving extension can quickly become unsustainable over time. In addition, a lack of automated validation can have serious patient safety implications.