When values within a SNOMED CT expression, expression constraint or query are unknown at the time of authoring, a SNOMED CT template may be used. The SNOMED CT template syntax can be used in conjunction with the SNOMED CT computable languages to create SNOMED CT template languages for this purpose. For example:

• Using the template syntax with SNOMED CT compositional grammar enables the representation of expression templates

• Using the template syntax with the SNOMED CT expression constraint language enables the representation of expression constraint templates

• Using the template syntax with the SNOMED CT query language (1) enables the representation of query templates.

This version of the SNOMED CT template syntax specifically focuses on supporting two important use cases in which the SNOMED CT template syntax is needed. In particular to support:

These use cases specifically require the use of slots within SNOMED CT Compositional Grammar. Therefore, this version of the specification focuses primarily on supporting the use of the template syntax within the context of the . Future versions of this specification will explore other use cases, and additional template languages (e.g. the Expression Constraint Template Language).

Expression templates can be used to author precoordinated clinical meanings by providing reusable patterns for the definition of similar concepts. Expression templates can define the set of attributes and attribute values that may be used to define concepts in a specific hierarchy or subhierarchy, thereby assisting SNOMED CT authors to create concepts in a consistent way. They can also be used together with an input file to either create or update batches of concepts very efficiently. Using expression templates to author SNOMED CT concepts can help to improve the quality of the terminology, by controlling the consistency of the definitions, and by validating concept definition against appropriate templates. Similarly, they can also be used as a metric to measure the quality of given subhierarchy.

In summary, expression templates can be used by terminology authors to:

• Create a single concept, or a batch of concepts;

• Update the definition of a concept (or batch of concepts) to conform to a particular pattern;

• Validate the definition of concept or a set of concepts;

• Define quality metrics for a particular subhierarchy.

The example below shows an expression template that may be used to define computed tomography (CT) concepts of a particular body site (e.g. CT of right arm). The 'bodysite' slot represents the placeholder, which is subsequently replaced with a specific body site concept to form a complete concept definition.

Example: An example expression template

The following examples show the expressions that result when processing the template using a specific slot value:

Example: Expressions that result from populating the example expression template

Expression templates can be used to enable the easy and consistent authoring of postcoordinated expressions. They are particularly useful in situations where a specific pattern is needed to support the entry, storage or retrieval of SNOMED CT expressions. Therefore, expression templates are useful for:

• Creating batches of postcoordinated expressions with a consistent structure; and

• Composing postcoordinated expressions from data entered into a user interface.

These uses are explained in more detail in the following sections.

Batch Authoring of Expressions

Expression templates may be applied to ensure a consistent structure is used by a set of authored expressions. For example, there may be a preference to always represent allergies using an expression in which the allergen is explicitly defined as the value of the attribute. Given an appropriate expression template and a predefined list of allergen substances, a set of postcoordinated expressions can automatically be batch authored (as shown below).

Expression Template

: = [[+id @Substance]]

List of Substances

Resulting Expressions

: = : = : = : = : =

Expression Templates are also useful for creating postcoordinated expressions from data entered in a user interface. For example, a radiology user interface may use two separate fields to capture the imaging procedure and the body site to which the procedure was applied. An expression template can then be applied to combine the data entered into these two fields into a single postcoordinated expression (see diagram below).