When postcoordinated expressions are used to represent clinical information, this data needs to be queried and analyzed in the same way as data recorded using precoordinated concepts.
Systems supporting postcoordination should thus ensure that analytics features are enabled to take advantage of SNOMED CT's hierarchy and explicit representation of defining properties. Furthermore, this facility should allow queries to be generated that combine SNOMED CT-specific selection criteria with other health record criteria.
Key analytics features that should be supported include:
- Subsumption which involves the determination of whether one concept (or expression) is a kind of another concept (or expression) is the fundamental capability enabled by SNOMED CT. For example, answering the question 'Which patients have an infectious disease?' involves finding all the patients with any kind of infectious disease (e.g. viral pneumonia, tuberculosis).
- Selective retrieval which involves using the properties (relationships) derived from classification to retrieve concepts (or expressions) matching specified criteria.
Subsumption
Subsumption testing between expressions tests to see if the candidate expression (often recorded in a patient record) is subsumed by a predicate expression (typically part of the query being run across the patient record). To enable subsumption testing it is necessary to ensure that the expression is processed in the same way as the precoordinated content. This requires the expression to be positioned in the right coordinates of the SNOMED CT hierarchy (aka coordinated), as illustrated in Figure 4.5-1.
Consider the expression:
Subsumption testing may be performed to test if this expression is a subtype of the concept
300848003 |Mass of body structure (finding)|
. This test could be required to identify a cohort of patients who have been diagnosed with some kind of lump in a body structure.
However, to enable systems to perform this test, the expression first needs to be classified with the precoordinated content to generate an inferred subtype hierarchy including both the postcoordinated expression and the query concept. If a classification is not performed, the software will not be able to automatically derive where the expression belongs in the SNOMED CT hierarchy relative to the query concept.
As illustrated in the image below, the candidate expression is subsumed by the predicate expression.
Figure 4.5-1: Subsumption testing of expressions requires the expressions to be processed as if they were part of the released (precoordinated) SNOMED CT content.
Selective Retrieval
Systems supporting postcoordination should provide a mechanism to allow users to specify retrieval requirements based on the inferred properties of an expression. Attribute relationships in conjunction with the subtype hierarchies provide an efficient mechanism for specifying query criteria, e.g. a query for "all diseases with a | Finding site| of | Shoulder region structure| and an | Associated morphology| of | Edema| ". As illustrated in Figure 4.5-2, the inferred properties can be used to identify the concepts and expressions matching these query criteria. However, as for subsumption testing, this kind of query feature demands for the expressions to be classified with the precoordinated concepts in order for the expressions to infer all necessary properties correctly.
Figure 4.5-2: Illustration of the principles of how the inferred properties of concepts and expressions can be utilized to specify query criteria selectively.
Terminology services
Enabling queries over expressions to be performed requires the availability of the following terminology services:
- 5.2.10 Query Expression - Including both services to perform subsumption testing and selective retrieval of expressions
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