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SNOMED CT is a clinical terminology with a comprehensive scope covering a wide range of clinical specialties and requirements. The SNOMED CT Concept Model is a set of rules that govern the ways in which SNOMED CT concepts are permitted to be modelled using relationships to other concepts.
These rules are critical to the consistent modelling of SNOMED CT content, which in turn determines the extent to which reproducible logical inferences can be drawn. These logical inferences are the foundation for effective use of SNOMED CT for retrieval and reuse of clinical information.

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.

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In 2007, the IHTSDO approved a project to develop a prototype MRCM for SNOMED CT. In 2009, a MRCM draft release 0.1 was published containing 234 constraints, including domain, range and cardinality constraints. This draft MRCM was released in two formats - an XML representation, and a set of MS-Access 2007 database files. It also came with a prototype MRCM browser and editor written in MS-Access.

Since then, two important developments have occurred. Firstly, in 2012 a new standard distribution format for SNOMED CT (Release Format 2) began to be used. Release Format 2 (RF2) included a few key enhancements, including more robust and consistent version representation, an extensibility mechanism called 'reference sets', and a new hierarchy to represent metadata about the structure of SNOMED CT. Secondly, in 2015, the first official version of the Expression Constraint Language was published, replacing earlier informal constraint representations.

The combination of RF2 reference sets and SNOMED CT Expression Constraints provide an opportunity to define the SNOMED CT MRCM in a more standardized, consistent and version controlled way than was previously possible.

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The purpose of this document is to define and describe the proposed structural design of the SNOMED CT MRCM, and to discuss the background and considerations that were taken into account during its design. This document is intended to be read in conjunction with a preview of the current SNOMED CT Concept Model, represented using this draft design.

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This document presents the specification of the SNOMED CT MRCM, including the reference sets and attributes used to represent and distribute the international SNOMED CT MRCM. It also documents the background, use cases, requirements and design considerations, including versioning and extensibility of the MRCM.

While the document refers to the RF2 format and the Expression Constraint Language, the specification of these standards are out of scope. Additionally, this document does not go into details regarding about how to implement the MRCM within a terminology authoring or Electronic Health Record environment.

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