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As a consequence of emerging clinical needs and general terminology maintenance and enhancement, terminology producers will be required to author clinical meanings to be included as precoordinated concepts in either the International Edition of SNOMED CT or a SNOMED CT Extension.

Expression templates are reusable patterns for the precoordination of clinical meanings, because they can specify the restricted set of attributes and attribute ranges used in the creation of specific concepts. SNOMED CT expression templates can therefore assist SNOMED CT authors in creating concepts with predefined constraints, and they can enable the creation of batches of concepts or their updates.

Using templates to author SNOMED CT concepts helps to improve the quality of the terminology because templates can support the consistent definition and representation of concepts of the same type. As a result, expression templates can also be used a metrics for the measurement of quality.

In summary, Expression Templates can be used by authors to:

create a single concept
update an existing single concept to conform to a particular template/pattern
create a batch of new concepts
update a batch of existing concepts

The illustration below shows how a template can be created to specify how to define concepts that represent the CT (Computed tomography) of a certain bodysite, for example the CT of right arm or the CT of left leg. The @bodysite slot represents the variable which can be replaced by certain concepts or expressions to form the definition of a new concept.

Template (diagram)

Template (syntax)

71388002 |Procedure|:
   {260686004 |Method| = 312251004 |Computed tomography imaging action|,
        363704007 |Procedure site| = [[+id @bodysite]] }

The following table shows the resulting expression when processing the template using a specific slot value:

Slot Value (@bodysite)

Resulting Expression

71388002 |Procedure|:
   {260686004 |Method| = 312251004 |Computed tomography imaging action|,
        363704007 |Procedure site| = 48979004 | Structure of left lower leg (body structure) | }

71388002 |Procedure|:
   {260686004 |Method| = 312251004 |Computed tomography imaging action|,
        363704007 |Procedure site| = 368209003 | Right upper arm structure (body structure) | }

Alternative representation of the same example:

If the following values are provided to complete the slot:

then the expressions that result from processing the template would be:

71388002 |Procedure|:
   {260686004 |Method| = 312251004 |Computed tomography imaging action|,
        363704007 |Procedure site| = 48979004 | Structure of left lower leg (body structure) | }

71388002 |Procedure|:
   {260686004 |Method| = 312251004 |Computed tomography imaging action|,
        363704007 |Procedure site| = 368209003 | Right upper arm structure (body structure) | }