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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 subsectionssections, we describe four of the main use cases for SNOMED CT Compositional Grammar, including:

• Expressions in health records;
• Expressions in messages;
• Precoordinated concept definitions; and
• Expression associations between SNOMED CT and LOINC.