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SNOMED CT, enables the use of postcoordinated expressions to represent detailed clinical information (such as observations or procedures) by reference to multiple Concept Identifiers .
When considering migration of existing data an important question is whether postcoordination is required to replace existing coded data. The answer to this question depends on the specificity and expressivity of the existing coding scheme.
There are four situations in which postcoordination may be required or useful in the mapping process.

  • To capture data postcoordinated in the original coding scheme:
    • The extent to which this data can be mapped to SNOMED CT depends on the consistency of the original representation and the degree of alignment with the SNOMED CT Concept Model. Refinements in the source data are not sanctioned by the Concept Model may be mapped to similar approved Attributes or downgraded to text. Alternatively, they may be retained as expressions that fall outside the scope of Concept Model although this may limit effective retrieval.

Example: Information coded using NHS Clinical Terms Version 3, may also include postcoordination using qualifiers. In most cases, these qualifiers can be represented using postcoordinated expressions .

Example: A specialized radiology coding systems may have separate codes for the same procedure applied to different body sites. If SNOMED CT does not include these, they can be represented using postcoordinated expressions with 'procedure site' and 'laterality' Attributes applied.

  • To incorporate additional information which the originating clinical system represents in a consistent proprietary form.

Example: A system may have a separate field for laterality and this can be applied during the mapping process to generate a postcoordinated expression .

  • To satisfy a preference for a consistent postcoordinated representation of a particular type of data.

Example: There may be a preference to always represent allergies by postcoordinating the substance, rather than using one of the precoordinated 'allergic to x' concepts .

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