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The SNOMED CT Composition Grammar was initially specified as part of the document "SNOMED Clinical Terms Abstract Logical Models and Representational Forms, External Draft for Comment Version". This version (v1) was used extensively as both a human readable and machine parsable syntax.

A revised version (v2.0 to v2.2) of this specification was adopted as an IHTSDO standard in 2010. This version followed the prior version in most details, with the following enhancements:

• The specification of the grammar was defined in Augmented Backus-Naur Form (ABNF)

  Footnote Macro
  ABNF as defined by Internet Standard 68, RFC 5234

  . This provided a formal standards-based reference for the grammar's structure.

• Unnecessary whitespace designators (i.e. <ws>) were removed from several places in the grammar.

• A maximum length constraint for SNOMED CT identifiers  (

  Gloss
  t SCTIDs GlosTerm SNOMED CT Identifier

  ) was added.

  Gloss
  t SCTIDs GlosTerm SNOMED CT Identifier

   consist of a sequence of digits, which must be between 6 and 18 digits in length.

• The hex code for carriage return (CR) was incorrectly given as '0C' in the previous version. It was corrected to '0D' in the new version.
• Detailed character encoding information for  UTF-8 was added.
• The ABNF definition of  term was amended to allow correct parsing by the APG ABNF parser generator.

In 2015, the original IHTSDO standard was revised (v2.3 and v2.3.1). The new specification is completely backwardly compatible with the prior standard, in that any expression written using the prior standard will necessarily conform to the new standard. However, two significant enhancements were made:

• Concrete values (e.g. integers, decimals and strings) are now permitted as attribute values.
• A definition status may now (optionally) be included at the start of an expression to indicate whether the clinical idea being expressed is 'equivalent to' or a 'subtype of' the expression.

In 2020, concrete attribute values were extended to permit booleans (v2.4).

These enhancements will be described and explained further within this guide.

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The purpose of this document is to define and describe a formal compositional grammar which is used to represent SNOMED CT expressions. SNOMED CT expressions are a structured combination of one or more concept identifiers used to express a clinical idea. SNOMED CT expressions may exist either independently or within the scope of an information model, message instance, EHR system or clinical repository. This document also provides examples and guidance to assist in the implementation of this syntax. 

Scope

his This document presents the specification of a compositional grammar, which can be used to represent SNOMED CT expressions. This document also provides a logical model for the grammar, discusses a set of example expressions and describes some implementation considerations.

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• Expression Constraint Language: used to define a bounded set of clinical meanings represented by either precoordinated or postcoordinated expressions; Query Language: designed to express computable queries over SNOMED CT content; and
• Templates: which allow slots to be added to expressions, expression constraints or queries, which can be filled with specific values at a later time.

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The target audiences of this document include:

• IHTSDO SNOMED National Release Centers;
• SNOMED CT designers and developers, including designers and developers of EHR systems, information models, data entry interfaces, storage systems, decision support systems, retrieval and analysis systems, communication standards and terminology services;
• SNOMED CT terminology developers, including concept model designers, content authors, map developers and release process managers.

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