A new version of the International Edition of SNOMED CT is released twice a year (in
January and July). National extensions mostly follow this cycle (albeit typically
with a three month delay). However, some extensions (notably those including medication
related concepts) are released more frequently.
When a longitudinal health record is populated with clinical data over a number of
years, it is quite possible that the following may occur:
- SNOMED CT concepts that were active at the time of recording have since been made
- SNOMED CT concepts that were primitive at the time of recording have since been defined
- Reference sets that were used to populate pick lists may have changed
- The SNOMED CT Concept Model that was used to construct expressions may have changed
To mitigate these versioning issues, SNOMED CT provides the following:
- Each new version of the SNOMED CT International Edition that is released (in Release
Format 2 -RF2) includes a set of Delta files (containing all changes to the content
since the last release), a set of Snapshot files (containing the most recent version
of every component that has ever been released in SNOMED CT), and a set of Full files
(containing every version of every component that has ever been released in SNOMED
CT). These files allow implementations to either incrementally adapt to new versions
of SNOMED CT, or alternatively load a complete current snapshot of SNOMED CT content
(with or without old versions). When longitudinal clinical records containing inactive
concepts are queried, all prior descriptions and relationships of these inactive concepts
can still be queried using these snapshot files. SNOMED CT's RF2 distribution files
also record the reason that each inactive component was inactivated, using 'historical
association' reference sets (see
4.2.3 Historical Association Reference Sets for more details).
- SNOMED CT is maintained on the principle that every SNOMED CT concept identifier should
retain its semantic integrity over time, even when its logical definition changes.
The semantics of a SNOMED CT concept is established through its Fully Specified Name,
and all changes to a concept's defining relationships are intended to improve the
machine-readable processing of these semantics. That said, it is possible if required
to determine what the logical definition of a concept was at any prior point in time
using a Full release of SNOMED CT.
- SNOMED CT's reference sets and their members are all fully versioned in SNOMED CT's
RF2. A Snapshot release of a reference set includes the current version of every row
that has ever been released (including both active and inactive rows). A Full release
of a reference set includes every version of every row that has ever been released.
Using this information, it is possible to adapt queries to consider both current and
former members of any given reference set.
- The SNOMED CT Concept Model changes very rarely. When it does, however, any attributes
that are retired are retained as inactive concepts in the Snapshot and Full releases
of SNOMED CT. It is expected that a complete Machine Readable Concept Model (MRCM)
of SNOMED CT will be published in the future, and that this MRCM will be versioned
in a manner that is consistent with other RF2 components.