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SNOMED CT enables the use of postcoordinated expressions to represent detailed clinical information (such as observations or procedures) by reference to multiple Concept Identifiers. Expression templates can be used to enable the easy and consistent authoring of postcoordinated expressions and . They are particularly useful in situations where a specific pattern is needed to support the entry, storage and or retrieval of SNOMED CT expressions.  Hence Therefore, expression templates are useful for:

• creating Creating batches of postcoordinated expressions with a consistent structure; and
• Composing composing postcoordinated expressions from data entered into a user interface.

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

Batch

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Authoring of

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Expressions

Expression templates may be applied to satisfy a preference for a consistent postcoordinated representation of a particular type of dataensure a consistent structure is used by a set of authored expressions. For example, there may be a preference to always represent allergies by postcoordinating the disorder with the substance, rather than using one of the precoordinated 'allergic to x' concepts . In this situation,  given a predefined set of substances, an expression template can support the batch authoring of postcoordinated expressions and thus represent the concerned allergic disorders consistently, as illustrated below.using an expression in which the allergen is explicitly defined as the value of the

419199007 | allergy to substance |: 246075003 | causativeCausative agent | = [[ +id @Substance ]]

256259004 |Pollen (substance)|
89811004 |Gluten (substance)| 
47703008 |Lactose (substance)|
13577000 |Nut (substance)|
33396006 |Nickel (substance)|

419199007 | allergyAllergy to substance |: 246075003 | causativeCausative agent | = 256259004 | Pollen (substance) |
419199007 | allergyAllergy to substance |: 246075003 | causativeCausative agent | = 89811004 | Gluten (substance) |
419199007 | allergyAllergy to substance |: 246075003 | causativeCausative agent | = 47703008 | Lactose (substance) |
419199007 | allergyAllergy to substance |: 246075003 | causativeCausative agent | = 13577000 | Nut (substance) |
419199007 | allergyAllergy to substance |: 246075003 | causativeCausative agent | = 33396006 | Nickel (substance) |

Composing

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Expressions from

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a User Interface

Expression Templates are also useful for creating postcoordinated expressions from data entered using in a simple user interface. For example, a specialized radiology coding system may have the option user interface may use two separate fields to capture the radiology imaging procedure and the body site to which the procedure was applied. In this situation, an An expression template can then be defined applied to specify the syntactically valid expression for a specific radiology procedure with a given direct procedure site combine the data entered into these two fields into a single postcoordinated expression (see diagram below).