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Definitions and qualifiers in SNOMED CT.

OBJECTIVES
An important feature of SNOMED CT is post-coordination, which is enabled by the SNOMED CT representation specifying whether a relationship is a defining or a qualifier relationship. In this paper the use of qualifier relationships in SNOMED CT is analyzed, as well as the extent to which qualifiers interact with defining relationships, so that pre-coordinated concepts can also be post-coordinated.


METHODS
The July 2007 release of SNOMED CT was imported into a database. Analyses were performed by querying this database.


RESULTS
Qualifier relationships occur in 10 out of 61 types of attribute relationships, and it is shown that generally pre-coordinated concepts cannot be constructed by applying post-coordination using qualifier relationships. Most of the qualifier relationships have generic target concepts, making it possible to construct concepts which are not clinically sensible. A logic-based representation is proposed to overcome the drawbacks of the current model.


CONCLUSIONS
Defining and qualifier relationships both enable post-coordination in SNOMED CT. Introducing qualifiers for more types of relationships, and using qualifier relationships with more specific target concepts will further improve post-coordination in SNOMED CT.