Purpose

The | Code to expression type reference set|  is designed to enable associations between code in another code system (other-codes) and SNOMED CT concepts, where the following constraints apply:

1. Some of the other-codes cannot be mapped to an individual SNOMED CT concept.
2. Licensing conditions (or other considerations) prevent addition of new SNOMED CT concepts to represent the same meaning as the other-codes.
3. The other-codes can be logically defined using the SNOMED concept model to represent the same meaning (sufficiently defined) or a similar though less specific meaning (primitive).
4. Other requirements similar for those applicable to mapping may also apply including:
   1. An indication of the degree of correlation between the  other-code and the SNOMED CT expression.
   2. An indication of whether the other-code was created before any single concept representation of that meaning in SNOMED CT or whether the single concept representation in SNOMED CT predated the creation of the association.

Data Structure

The general approach to the above requirements is to associate each of the other-codes with a representation of the same logic based definition as would have been applied to a SNOMED CT concept with that meaning. However, since the  other-code are not identified by an  SCTID, the logical definition cannot be represented using  defining relationships. There are two potential approaches to this, one would be to use a general purpose description logic language (e.g.  OWL) and the other is to use a SNOMED CT expression to represent each definition. The | Code to expression type reference set|  is designed to support the expression-based approach.