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SNOMED CT
, enables the use of
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postcoordinated expressions
to represent detailed clinical information (such as observations or procedures) by reference to multiple
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Concept Identifiers
.
When considering migration of existing data an important question is whether
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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
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postcoordination
may be required or useful in the mapping process.

  • Anchor
    _625cddf6-ba29-44ac-9a70-0804441bd721__1
    _625cddf6-ba29-44ac-9a70-0804441bd721__1
    To capture data
    Glossary ref
    postcoordinated
    in the original coding scheme:
    Anchor
    _625cddf6-ba29-44ac-9a70-0804441bd721__d
    _625cddf6-ba29-44ac-9a70-0804441bd721__d
    • The extent to which this data can be mapped to
      Glossary ref
      SNOMED CT
      depends on the consistency of the original representation and the degree of alignment with the
      Glossary ref
      SNOMED CT Concept Model
      . Refinements in the source data are not sanctioned by the
      Glossary ref
      Concept Model
      may be mapped to similar approved
      Glossary ref
      Attributes
      or downgraded to text. Alternatively, they may be retained as
      Glossary ref
      expressions
      that fall outside the scope of
      Glossary ref
      Concept Model
      although this may limit effective retrieval.

Example: Information coded using

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

  • To represent information which the originating coding system precoordinates but which
    Glossary ref
    SNOMED CT
    can only represent using a
    Glossary ref
    postcoordinated expression
    .

Example: A specialized radiology coding systems may have separate codes for the same procedure applied to different body sites. If

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SNOMED CT
does not include these, they can be represented using
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postcoordinated expressions
with 'procedure site' and 'laterality'
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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

Glossary ref
postcoordinated expression
.

  • To satisfy a preference for a consistent
    Glossary ref
    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

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precoordinated
'allergic to x'
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concepts
.