The transition to SNOMED CT for users of SNOMED RT is relatively straightforward because the Concept Identifiers of SNOMED RT are for the most part the same as those used in SNOMED CT. In some cases, during the merger of SNOMED RT and Clinical Terms Version 3 some Concepts in SNOMED RT have been found to be ambiguous or duplicated. These Concepts have been inactivated by an appropriate change of status and adding a record in the Component Inactivation Reference Sets, but are still present in the Concepts Table and are linked to Active Concepts by the Historical Associations Reference Set:
- Each duplicate Concept has a | SAME AS | Association to the Active Concept with the same meaning as the duplicate Concept;
- Each ambiguous Concept has | MAY BE A | Association to one or more Active Concepts, which represent possible disambiguated meanings.
These Associations can be used either to allow Concepts recorded using these Concepts to be recognized by retrieval tools or to enable mapping of the stored information to the appropriate active Concept Identifier.
If any stored Concept Identifier of an Inactive Concept is mapped to an active Concept Identifier using these Associations it is strongly recommended that the original Concept Identifier is also retained. This enables future improvements or corrections of such mappings if revised Associations are present in a future release of SNOMED CT.
In addition, SNOMED RT contained both generic and brand name drugs for the US. A decision was made during the merger process to not retire these concepts using the extension mechanisms, but to place these components directly in the US Drug Extension. Therefore to access all SNOMED RT components you will need to use the US Drug Extension in addition to the SNOMED CT International Release.