A core responsibility of an NRC is to make the International Edition of SNOMED CT and/or the National Extension of SNOMED CT available for Affiliates.

Every NRC must therefore be able to provide guidance on access to SNOMED CT, version control and management. Moreover, the NRC should be able to provide guidance to users on specific terminology services that are provided by the NRC, and in the use of national SNOMED CT developments, such as National Extensions, Reference sets, etc.

SNOMED International maintains and delivers shared content and derivatives¹ that provide the foundation of SNOMED CT. This is known as the SNOMED CT International Edition.

SNOMED CT is distributed as sets of files containing terminology components and derivatives. These contain the basic data needed to implement SNOMED CT in various modes in a technology-neutral formats. The format, content and names of the files delivered conform to SNOMED CT specifications and guidelines published in SNOMED CT Release File Specification.

Components represent the content of the terminology.

The standard SNOMED CT representation for content is as three interrelated files. The Concept file contains unique identifiers for clinical ideas, the Description file links human readable terms with identified concepts, and the Relationship file represents associations between identified concepts.

Derivatives facilitate the effective use of the terminology.

The standard SNOMED CT representation for derivatives is a consistent but flexible file format, known as the Reference set format. Reference sets can be used for a wide range of purposes including subsets, language preferences, ordered lists, hierarchies, annotations and mappings to or from other terminologies, classifications and code systems.

Access to SNOMED CT release files:

Members of SNOMED International and Affiliates outside of Member countries, both access the release files via SNOMED International's SNOMED CT Licensing and Distribution Services (MLDS).

All files distributed to Members are available from the "SNOMED CT International Edition" product page on MLDS, and can be accessed here: https://mlds.ihtsdotools.org/#/ihtsdoReleases/ihtsdoRelease/167

Access and information about MLDS can be found here: https://mlds.ihtsdotools.org.

Release Lifecycle

SNOMED International provides regular updates to the SNOMED CT International Edition. SNOMED CT is released monthly on the first day of each month. Further details can be found in https://www.snomed.org/releases.

Similarly, Members and Affiliates that maintain Extensions should release updated versions of their Extensions to align with the SNOMED CT International Edition.

Members' Release

Regular changes to SNOMED CT are necessary in order to satisfy user needs and requirements and to improve the quality of SNOMED CT itself. An NRC must be aware that any change to SNOMED CT may have impact on National Extensions and guidance. Therefore, every release requires reconciliation to ensure that all National Extensions remain aligned with the International Edition.

If SNOMED CT components (concepts, descriptions or relationships) are added or inactivated, an assessment of how these changes affect a National Extension is be required, and adjustments to the national developments may be needed. For example:

• Added concepts may require translation and may also be candidates for addition to national Reference sets.
• Inactivated concepts that members of a National Reference set may need to be replaced with an appropriate active concept.

Release files

With every SNOMED CT release two different release file types are made available:

• Full Release: containing the complete history of every component
• Snapshot Release: containing the current state of every component

Note on Delta Release (includes changes made between one version and the next): Delta files have been removed from the SNOMED International release package, Managed Service Extensions and Derivative packages. However a Delta Generation Tool is available for those who need it. The Delta Generation Tool allows users to create their own Delta between two fixed release dates - you can find it here: https://github.com/IHTSDO/delta-generator-tool/releases.

The International Edition is provided in both release types. However, as the Snapshot can be generated from the Full Release, the specification only requires that organizations that maintain Extensions provide a Full Release of their extension.