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SNOMED CT is delivered as sets of files containing terminology components and derivatives. The format, content and names of the files delivered conform to SNOMED CT specification and guidelines published by the IHTSDO.

  • Components represent the content on 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 mapping to or from other terminologies, classifications and code systems.

IHTSDO maintains and delivers shared content and derivatives that provide the foundation of the SNOMED CT. This is known as the SNOMED CT International Edition.
IHTSDO also authorizes Members and Affiliates to maintain and deliver additional components and derivatives known as SNOMED CT Extensions.

  • Any organization maintaining an Extension needs to have a namespace identifier allocated to them by the IHTSDO. Every component created must be allocated a new permanent SNOMED CT identifier which is in the originating organization's namespace allocated. The namespace identifier of the originating organization forms part of every component identifier allocated.

IHTSDO Members may maintain and deliver additional terminology components and derivatives that adapt the terminology to meet specific national requirements (National Extension).
A National Extension may include:

  1. translation into the national language or adaption to a national dialect;
  2. additional content to support national policy objectives, a national drug dictionary or other specific requirements;
  3. derivatives that configure use of SNOMED CT content by specifying subsets of content to be used for particular purposes;
  4. derivatives that map other code systems used in that country to or from SNOMED CT.

IHTSDO Affiliates may also maintain and deliver additional terminology components and derivatives that adapt the terminology to meet the needs of a particular organization, customer or software solution (Affiliate Extension).
Examples: An Affiliate Extension may include:

  1. additional content enable a health provider organization or clinical specialty group to address its priority use cases;
  2. derivatives that configure use of SNOMED CT in ways that reflect the needs of a health provider organization or specialty;
  3. derivatives that configure the way SNOMED CT is used or presented to different customers using a particular software applications;
  4. derivatives that map local or proprietary code systems to or from SNOMED CT.

The SNOMED CT International Edition can be used without any Extensions. However, a SNOMED CT Extension cannot be used on its own because all Extensions are dependent on the International Edition and some Extensions are also dependent on other Extensions. Therefore, for each Extension there is a corresponding Edition that includes the Extension, the International Edition and any other Extensions on which it depends.

  1. Healthitia, an IHTSDO Member, produces the Healthitia National Extension. Affiliates in Healthitia use the Healthitia National Edition (the International Edition and the Healthitia National Extension).
  2. Anyville Hospital in Healthitia is an IHTSDO Affiliate. It produces its a local Anyville Extension for use in the hospital and this Extension also depends upon the Healthitia National Extension. Therefore, the Anyville Edition consists of the International Edition, the Healthitia National Extension and the Anyville Extension;
  3. AcmeGest an international provider of antenatal care systems is an IHTSDO Affiliate and uses the AcmeGest Extension to support some of its specialist functionality. This Extension only depends on the International Edition, therefore the AcmeGest Edition consist of the International Edition and the AcmeGest Extension.
  4. The obstetric department of Anyville Hospital in Healthia uses the AcmeGest system., so it requires the AcmeGest Extension as well as the Anyville Edition. The AcmeGest Extension has a dependency on the International Edition but this should only be resolved once.

Figure 1. Illustration of the relationships between Extensions and Editions
For use in particular countries or institutions there may be advantages in distribution a complete Edition. However, as the last of example above illustrates, different Editions may share a common dependency on the International Edition. All organizations that maintain Extensions should make their Extension available as a separate set of files, even if they also provide a pre-merged Edition. This allow validation of the constituent parts of the Edition and also supports merges to produce bespoke Editions that combine several Extensions.
The IHTSDO provides regular updates of the SNOMED CT International Edition. Similarly, Members and Affiliates that maintain Extensions will from time to time release updated versions of their Extensions. Therefore,
The SNOMED CT release file specifications define three different release file types:

  • Full Release: containing the complete history of every component
  • Snapshot Release: containing the current state of every component
  • Delta Release (containing only the additions and changes since the previous release

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