The Expression Constraint Language is a formal syntax for representing SNOMED CT expression constraints. Expression constraints are computable rules used to define a bounded sets of clinical meanings represented by either precoordinated or postcoordinated expressions. Expression constraints can be used to restrict the valid values for a data element in an EHR, as the intensional definition of a concept-based reference set, as a machine processable query that identifies a set of matching expressions, or as a constraint that restricts the range of an attribute defined in the SNOMED CT concept model.

The has full details on ECL, including use cases, requirements, logical model, syntax specification, examples, and implementation considerations.

What is the Machine Readable Concept Model (MRCM)

The Machine Readable Concept Model (MRCM) represents rules in the SNOMED CT concept model in a form that can be read by a computer and applied to test that concept definitions and expressions comply with the rules. The MRCM may be used for a variety of purposes, including the authoring and validation of SNOMED CT concepts, expressions, expression constraints and queries, Natural Language Processing and terminology binding to support semantic interoperability.

Why is the MRCM needed?

The MRCM may be useful in a number of use cases, including:

• Development of precoordinated terminology content.

• Authoring and validation of SNOMED CT expressions, constraints and queries.

• Natural Language Processing.

• Terminology binding to information models, for purposes such as data capture and semantic interoperability.

The MRCM contains a set of domains and a set of attributes, which may be applied to one or more domains. The cardinality and valid range of each attribute is specified, and indicates whether or not the attribute should be grouped. The domain, range, cardinality and grouped indicator for each attribute is also combined into a single 'attribute rule', which is represented using a .

For each domain, the set of valid attributes and their associated rules are compiled into two SNOMED CT 'domain templates' – for precoordinated concept authoring and postcoordinated expression authoring respectively. These domain templates may be further specialized to support customized authoring of specific subdomains.

The following diagram summarizes the logical structure of the MRCM:

The MRCM is distributed as the following types of reference set in the SNOMED CT RF2 release file format:

• MRCM Domain Reference Set

• MRCM Attribute Domain Reference Set

• MRCM Attribute Range Reference Set

• MRCM Module Scope Reference Set

The following diagram summarizes these reference sets and their properties:

The Public MRCM Maintenance Tool supports browsing, filtering and concept matching to the Machine Readable Content Model (MRCM) constraints which apply to SNOMED CT concepts.

Key uses include:

• Finding high level concept Domains and their constraints

• Finding Applicable Attributes for Domains

• Finding Attribute Ranges for Applicable Attributes