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SNOMED CT support of the postcoordination technique allows additional clinical detail to be represented if required. For example, |pneumococcal pneumonia| has a |finding site| of |lung structure|, which can be refined to |right upper lobe of right lung|.

Postcoordination greatly increases the depth of detail that SNOMED CT can represent without having to include every possible specific site for every possible disorder via a concept. For example, the concept |bacterial pneumonia| has a defining relationship specifying its |causative agent| as |bacteria| and this can be refined to |Streptococcus streptococcus pneumoniae|.

SNOMED CT expressions are a structured combination of one or more concept identifiers used to represent a clinical idea in a logical manner, which is automatically processable. Expressions are represented using the SNOMED CT compositional grammar, which is a lightweight syntax for the representation of SNOMED CT expressions.

The logic on which the SNOMED CT concept model is based allows alternative representations of the same or similar information to be recognized and compared. For example, |pneumococcal pneumonia| refined by |finding site| |right upper lobe of right lung| can be computed to have the same meaning as |right upper lobe pneumonia| refined by |causative agent| |Streptococcus streptococcus pneumoniae| and addition of |pathological process| |infectious process|.

Precoordinated Expressions


The second example shown below illustrates the fact that some SNOMED CT concepts provide quite a lot of detailed refinement, some of which might otherwise be captured separately. We will return to this example when considering postcoordination.

Example:  Precoordinated representation of "Laparoscopic emergency appendectomy"
SNOMED CT contains the concept 174041007 |laparoscopic emergency appendectomy|. The identifier of this concept (174041007) can be used (with or without the associated term) as a precoordinated expression to record an instance of this procedure.
The procedure 'laparoscopic emergency appendectomy' has at least three distinct facets: '


excision of appendix', 'using a laparoscope' as 'emergency procedure'. The

174041007 |laparoscopic emergency appendectomy| precoordinates these facets as its definition includes the following defining relationships:

  • 116680003 |is a| = 80146002| appendectomy|
  • 260870009 |priority| = 25876001 |emergency|
  • 425391005 |using access device| = 86174004 |laparoscope|

Postcoordinated Expressions


The example above shows that postcoordination can be applied even when a single concept is available to represent the required meansmeaning. However, the real strength of postcoordination is that it allows a clinical phrase to be represented even when the precise concept is not present in SNOMED CT. In these cases, postcoordinated refinements can be applied to an existing concept to more precisely capture the required meaning.


The basics of SNOMED CT compositional grammar

  • At its simplest level a single SNOMED CT concept identifier is a valid expression.
    • 80146002
  • A concept identifier can optionally be followed by a term associated with that concept enclosed between two pipe characters
    • 80146002 |appendectomy|
  • A concept identifier (with or without a following term) can be followed by a refinement. The refinement follows a colon
    • 80146002 |appendectomy| :<refinement>
  • A refinement consists of a sequence of one or more attribute-value pairs. Both the attribute and the value are represented by a concept identifier (with or without a following term). The attribute is separated from the value by an equals sign
    • 80146002 |appendectomy| : 260870009 |priority| = 25876001 |emergency|
  • If there is more than one attribute-value pair, the pairs are separated by commas
    • 80146002 |appendectomy| : 260870009 |priority| = 25876001 |emergency|, 425391005 |using access device| = 86174004 |laparoscope|
  • Curly braces represent grouping of attributes within a refinement, for example to indicate that the method applies to a specific site
    • 80146002 |appendectomy| : { 260686004 |method| = 129304002 |excision - action|, 405813007 |procedure site - direct| = 181255000 |entire appendix| }
  • Round brackets represent nesting to allow the value of an attribute to be refined
    • 161615003 |history of surgery| : 363589002 |associated procedure| = ( 80146002 |appendectomy| : 260870009 |priority| = 25876001| emergency| )

Postcoordination and the Concept Model


Example:  Postcoordination of family history
The definition includes 246090004 |associated finding| = 246090004 |disease|. This value can be refined to refer to a particular disease. For example:

  • 281666001 |family history of disorder| : 246090004 |associated finding| = 22298006 |myocardial infarction|

    The definition of |family history of disorder| specifies that the |subject relationship context| is |person in the family of subject|. This value indicates that the finding applies to a family member rather than the patient and can be refined to refer to a particular family member. For example:
  • 281666001 |family history of disorder| : { 246090004 |associated finding| = 22298006| myocardial infarction|, 408732007 |subject relationship context| = 44429500366839005 |father of subject| }

Consistent Retrieval of Precoordinated and Postcoordinated Expressions


Example:  Laparoscopic procedures
Several earlier examples in this chapter have represented procedures. If there was a requirement to retrieve all laparoscopic procedures the first step would be to establish the criteria for inclusion. The requirement could be expressed as follows.

  • 51316009 |laparoscopic procedure| and all its subtypes

    In some cases, it is simply a question of looking at the subtype hierarchy.
    For example
  • 174041007 |laparoscopic emergency appendectomy| is the source of a sequence of |is a| relationships which lead to 51316009 |laparoscopic procedure|.

    In other cases, it is necessary to look at the definition of |laparoscopic procedure| and compare this with the equivalent expression. |Laparoscopic laparoscopic procedure| is fully-defined as follows.
  • 71388002 |procedure| : 425391005 |using access device| = 86174004 |laparoscope|

    Therefore, queries looking for instances of laparoscopic procedures should also retrieve postcoordinated expressions in which both of the following are true:
  • The focus concept is a subtype of 71388002 |procedure|
  • The attribute 425391005 |using access device| is present with the value 86174004 |laparoscope| or a subtype of that concept.

    This rule would include postcoordinated representations, such as the following expression, for which there is no existing SNOMED CT concept:
  • 68526006 |removal of device from abdomen| : 425391005 |using access device| = 6174004 |laparoscope|