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The use and representation of hierarchies in SNOMED CT differs from the approach taken in many older code systems and classifications. This has a number of consequences that may affect the migration of queries, decision support protocols and data entry templates.

  • Meaningless Identifiers:
    • The codes specified in ICD-9, ICD-10, the Read Codes and SNOMED International provide information about where the code is located in a hierarchy. This allowed simple pattern matching queries to be used for some types of retrieval.

Example: In SNOMED International , all 'diagnoses related to the digestive system' can be retrieved by a query for all codes starting with 'D5'
The Identifier of a SNOMED CT concept does not provide any information about the way it relates to other concepts. Therefore, a simple pattern matching query cannot be used to retrieve related information represented using SNOMED CT. Instead, the query that specifies a subtype of the required concept is evaluated by testing the transitive closure of the set of subtype Relationships.
Example: All 'diagnoses related to the digestive system' can be retrieved by a query for expressions that are subtypes of 119292006 |disorder of gastrointestinal tract| .

Example: All 'diagnoses related to the digestive system' is some schemes may exclude codes that are primarily classified as infective disorders even they affect the digestive system. However, a SNOMED CT query includes all subtype concepts regardless of whether they are also in another hierarchy.

  • Different hierarchies:
    • Hierarchies in different code systems may be based on different principles and as a result queries that are migrated to SNOMED CT may return unexpected results. This may also relate to different interpretation of apparently identical concepts.

Example: Should a query for the concept 'nephrectomy' return only patients who have had a total nephrectomy or should the results include those with a record of a partial nephrectomy? If the partial nephrectomies are to be included then how much kidney tissue must be removed to count as a nephrectomy? If the partial nephrectomies are not included then should a record entry including the concept 'nephrectomy' (without specifying partial or total) be included? The SNOMED CT subtype hierarchy determines the answer to these questions based on the defining Relationships . The query may need to be refined to meet more specific expectations.

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