SNOMED CT Starter Guide

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  • Precoordinated Expressions
  • Postcoordinated Expressions

Why is this important?

SNOMED CT provides a mechanism that enables clinical phrases to be represented, even when a single SNOMED CT concept does not capture the required level of detail. This is important as it enables a wide range of clinical meanings to be captured in a record, without requiring the terminology to include a separate concept for every detailed combination of ideas that may potentially need to be recorded. Application software that supports the use of SNOMED CT expressions enables detailed clinical information to be recorded, retrieved and analyzed.

What is this?

Clinical expressions using SNOMED CT concepts can be of two types: precoordinated expressions, which use a single SNOMED CT concept identifier; and postcoordinated expressions, which contain more than one SNOMED CT identifier.

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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 lung| can be computed to have the same meaning as |right upper lobe pneumonia| refined by |causative agent||Streptococcus pneumoniae|.

Precoordinated Expressions

Precoordinated expressions are expressions that represent the meaning of individual concepts which are predefined in SNOMED CT. Besides the unique concept identifier and descriptions, each concept also has a formal logic definition represented by a set of defining relationships to other concepts. The figure below shows the precoordinated expression used to record |fracture of tibia|. It illustrates that this can be represented by a single identifier, with or without an accompanying human-readable term. It also illustrates the defining relationships of the concept identified in the expression. This is the precoordinated definitional knowledge which is conveyed by this expression.

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

Gloss
Spacetrue
tSNOMED CT
Gloss
tconcept
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

Expressions that contain two or more concept identifiers are referred to as postcoordinated expressions. Postcoordination combines concepts and allows more detail to be added to the meaning represented by a single concept. A postcoordinated expression is not just a list of concept identifiers, it follows a set of rules that mimic the way attributes and values are used to define SNOMED CT concepts.

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Postcoordinated expressions may be created at run-time by selection of individual facets of a concept. For example, to indicate the nature and location of a fracture for a particular bone and, where relevant, whether the bone affected is on the right or left. Some applications allow generation of postcoordinated expression using natural language processing. Alternatively, postcoordinated expression can be selected during user interface design and bound to simple data entry options. In these cases, the user may not be aware that the information is being captured in a postcoordinated form.

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Representing Postcoordination

There are several valid ways to represent and store postcoordinated expressions. However, to support interoperability, SNOMED International has specified a standard SNOMED CT compositional grammar form that is both human-readable and computer processable. The examples of expression that you see in this guide use this grammar.

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

The refinements used in postcoordinated expressions should follow the same concept model rules that are applied when concepts are defined. Attributes should only be applied to concepts that are in the specified 'domain' for that attribute. The values applied to attributes should be limited to the specified 'range' for that attribute. These rules may sometimes seem to limit flexibility but these rules are important, because if they are followed, it is possible to compute similarities and subtype relationships between different expressions. This ability to compute subtypes is the key to effective meaning-based retrieval of postcoordinated expressions.

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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 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|=444295003|father of subject|}

Consistent Retrieval of Precoordinated and Postcoordinated Expressions

SNOMED CT expressions support consistent and comparable representation of meaning using both precoordinated and postcoordinated expressions. This facilitates retrieval of all instances of expressions that match a set of criteria specified using the subtype hierarchy and other defining relationships.

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