SNOMED CT is a clinical terminology with a global scope covering a wide range of clinical specialties and requirements. Each concept in SNOMED CT represents a clinical idea to which a unique concept identifier has been assigned, but additional meanings can also be represented in SNOMED CT through expressions.
A SNOMED CT expression is a structured combination of one or more concept identifiers that represents a single clinical idea. A precoordinated expression is the simplest form of an expression. It contains a single concept identifier, and optionally one of the terms associated with the concept to make the expression human readable. A postcoordinated expression combines two or more concepts in accordance with rules expressed in the SNOMED CT Concept Model.
Postcoordination enables a wide range of clinical meanings to be captured in an EHR in a coded form, without requiring the terminology to include a separate concept for every detailed combination of ideas that may potentially need to be recorded.
Postcoordination supports a range of use cases, such as capturing clinical information in an EHR, sharing information across systems, supporting Natural Language Processing, and supporting mappings between SNOMED CT and other code systems.
Allthough, postcoordination may assist this range of use cases, the decision to use postcoordination requires careful consideration and preparation, because the implementation and support for expressions relies on the availability of human skills and dedicated tools supporting the safe creation, storage and processing of expressions.
Enabling postcoordination in an EHR requires an EHR design that is prepared to support the use of expressions in the same way as the released, precoordinated SNOMED CT content. Furthermore, it requires an EHR which uses a terminology server designed to enable the storage, processing and use of expressions.
Within a terminology server, the postcoordinated content lives alongside and is dependent on a specific versioned edition of SNOMED CT. The terminology server should, therefore, be designed to hold a given versioned SNOMED CT Edition, and an expression repository to hold the postcoordinated expressions. Furthermore, in addition to the services required by the EHR to use the content and derivatives of released SNOMED CT Editions, services are required to support the use of the postcoordinated content.
A terminology server may apply various levels of support for postcoordination. At the lowest level (level 0), the support for postcoordination involves the ability to accept only compositional grammar expressions that are fully compliant with the SNOMED CT Machine Readable Concept Model. At higher levels (level 1+), various patterns for non-mrcm compliant expressions are accepted and a higher degree of flexibility for the provided expressions are supported.
At present, it is recommended to only support postcoordination at either level 0 or 1, where the behavior and accepted expression types have been clearly specified. Further development is required to ensure that postcoordination at a higher level can be done without compromising clinical data quality.
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