Authoring of postcoordinated expressions may be conducted as part of an implementation project where a set of local interface terms or codes are mapped to SNOMED CT. Although it may be desired to map each local term to a precoordinated concept, mapping to expressions may be motivated by the following factors:
- An equivalent map is required but no precoordinated concept represents the semantics of the local term
- It is desired to map specific types of local terms to SNOMED CT using a consistent mapping structure
- For example, when mapping terms representing disorders of lateralized body structures, but only some of these disorders are precoordinated in SNOMED CT, then it may be chosen to map all of these cases to an expression of the same structure
Semantic Equivalence
When mapping to an expression it is important to aim for semantic equivalence, meaning using a focus concept and refinement that represents the full meaning of the local term. If this is not achievable, consider updating the local term to reflect the meaning of the expression.
Table 4.1.3.1-1 provides an example of two different local terms that are mapped to the same expression, and thus resulting in different semantic correlations.
| Source term | CTU Expression | Semantic correlation |
|---|---|---|
| Bleeding from larynx | Exact match | |
| Bleeding from larynx requiring transfusion | 131148009 |Bleeding| : 363698007 |Finding site| = 4596009 |Laryngeal structure| Note in this case the phrase "requiring transfusion" can't be modeled with the available attributes and, therefore, semantic equivalence is not achievable. | Narrow to broad |
Table 4.1.3.1-1: Semantic correlations between a source term and a target expression.
Alternatives to Mapping to a Postcoordinated Expression
Before deciding to map local codes or terms to SNOMED CT expressions, please consider the following alternatives:
- If you can't achieve an equivalent map from the local term to a precoordinated SNOMED CT concept, you may instead choose to map to a broader concept. Although this approach may lead to information loss because some of the meaning represented in the original local code will be degraded or lost in the map, the target concept may still retain enough of the critical original semantics required to support the subsequent use of the data
- If the local term is nationally or internationally relevant, a request for a new concept to be added to the national extension or the International Edition of SNOMED CT representing the meaning of the local term
- If the local term represents a meaning only just within the given country or organization, a concept may be added to the national/local extension representing the meaning of the local term
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