Use Cases
SNOMED CT Compositional Grammar provides a syntax that enables clinical ideas 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 analysed. The logic on which the SNOMED CT concept model is based allows alternative representations of the same or similar information to be recognised and compared.
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. Postcoordination greatly increases the depth of detail that SNOMED CT can represent, while avoiding a combinatorial explosion of precoordinated concepts.
In the following we describe four of the main use cases for SNOMED CT Compositional Grammar.
Expressions In Health Records
Clinical information recorded in EHRs using SNOMED CT is commonly represented using identifiers that refer to precoordinated concepts. These concepts may either be defined in the SNOMED CT international release or in an appropriate SNOMED CT extension. There are, however, times when a clinician needs to record and share a clinical meaning, which has not been defined in any release of SNOMED CT. In these situations, SNOMED CT postcoordinated expressions can be used to represent a new clinical meaning using the compositional grammar syntax.
Expressions in Messages
The SNOMED CT Compositional Grammar allows SNOMED CT expressions to be represented as a text string that can be carried in messages. For example, in HL7 version 3 messages SNOMED CT expressions can be placed in the 'code' property of the ConceptDescriptor (CD) data type (Release 2). The compositional grammar described in this document is intended to replace the qualifier mechanism that formerly was in the CD data type (Release 1), and which was removed in Release 2.
In September 2009, the HL7 Version 3 Standard "Data Types - Abstract Specification, Release 2" was published for its fourth normative membership ballot. This revised standard defined what can be carried in the 'code' property of the CD data type as shown.
Definition of the 'code' property of the Concept Descriptor (CD) data type from HL7 Data Types Release 2 Code (code) : ST.SIMPLE Definition: The plain code symbol defined by the code system, or an expression in a syntax defined by the code system which describes the concept. (emphasis added) If provided, the code SHALL be an exact match to a plain code symbol or expression defined by the codeSystem. If the codeSystem defines a code or expression that includes whitespace, the code SHALL include the whitespace.
An expression can only be used where the codeSystem either defines an expression syntax, or there is a generally accepted syntax for the codeSystem. (emphasis added)
The syntax described in this document is intended to satisfy the need for a "syntax defined by the code system" as stated above, when the "code system" is SNOMED CT. For specific guidance on using SNOMED CT Compositional Grammar in the CD data type (Release 2), please refer to Appendix B.4 of "HL7 Version 3 Implementation Guide: TermInfo – Using SNOMED CT in CDA R2 Models, Release 1".'
Precoordinated Concept Definitions
SNOMED CT Compositional Grammar may also be used to state the definition of precoordinated concepts. The introduction of definition status to this version of the language makes this usage more feasible. In particular, the definition of a fully defined concept uses an 'equivalent to' definition status to indicate that the expression is necessary and sufficient to define the clinical meaning of the concept. This is the same as giving the concept a definition status of 900000000000073002 | Defined| . The definition of a primitive concept uses a 'subtype of' definition status to indicate that the expression is necessary but not necessarily sufficient to define the clinical meaning of the concept. This is the same as giving the concept a definition status of 900000000000074008 | Primitive| .
For example, 31978002 | Fracture of tibia| (a fully defined precoordinated concept) can be defined using a compositional grammar expression as being 'equivalent to' an 428881005 | Injury of tibia| and a 46866001 | Fracture of lower limb| , with an 116676008 | Associated morphology| of 72704001 | Fracture| and a 363698007 | Finding site| of 12611008 | Bone structure of tibia| . This definition is shown below in Figure 1 using the SNOMED CT Diagramming Guidelines.
Please note that SNOMED CT expressions are used to represent a single clinical idea, rather than stating the equivalence between two clinical ideas. For this reason, a concept definition written as a SNOMED CT expression includes only the definition itself, without reference to the concept being defined.
For an extension to the grammar which includes both the concept and its definition together, please refer to Appendix A.
Postcoordination in SNOMED CT implementations
Expressions can be used to support SNOMED CT implementations in circumstances where the available pre-coordinated content does not fully meet the requirements for clinical representation. By employing postcoordination, existing concepts can be combined into a single expression that conveys a more precise or context-specific meaning.
For detailed guidance on the use of postcoordination within SNOMED CT, please refer to SNOMED CT Postcoordination Guide
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