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In one sense adding an expression to a repository and giving it an Identifier is the same as creating a new concept . However, there are some subtle but highly significant differences between storing, identifying and reusing an expression in the way suggested in the guide and a SNOMED CT concept . These are summarized in Differences between concepts and stored expressions .

Table 88. Differences between concepts and stored expressions


SNOMED CT released concept

Stored expression

The defining relationships of a SNOMED CT concept are intended to represent the meaning of words or phrases as they are used in clinical practice .

An expression is the collection of references to a set of SNOMED CT concepts .

The meaning of a SNOMED CT concept is represented by the Fully Specified Name (and sometimes by an associated textual definition).

An expression is not associated with a specific text string. It may be rendered in different human readable forms but its only source of meaning is the meaning of the concepts it references.

Because a concept definition attempts to express the human understood meaning of a word or phrase the logical definition expressed by its defining relationship may not be sufficient to fully define the concept . In these cases the concept definition is marked as " primitive ".

Because an expression has no specific term or source of meaning other than the focus concept and Attribute it is inherently " fully defined " in that those Attributes fully define what the expression may be used to represent.

A SNOMED CT Concept can be bound to various terms that are deemed to be synonyms in a given language . The SNOMED CT design provides a framework for managing these bindings, correcting errors, supporting translations and tracking the history of changes.

It may be tempting to associate particular words or phrases with an expression . This will inevitably occur in instances in individual record entries. However, terms should not be bound to expressions in a way that suggests a formal persisting association between that term and the class represented by the expression . If such a binding is required a SNOMED CT concept should be requested (or created in an extension ) to provide a proper framework for managing that binding.