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For example, Doctor Jones pre-operative order set should not be included because it is an individual instance, not a class.

Negation

Negation in SNOMED CT is generally out of scope for the terminology.  However, SNOMED CT currently has content which incorporates negation in several hierarchies. This is due to the amalgamation of legacy content from Read Codes and SNOMED RT in 2002 when the two terminologies formed SNOMED CT.  The description logic approach to negation comes with a high machine processing cost, and so the decision was to deal with all negation via the Situation with explicit context hierarchy.   The consequence of this approach is that the representation of negation within SNOMED CT that arises from restrictions imposed by the existing description logics results in the hierarchy being inverted, e.g., Coronary heart disease not present is not properly a subtype of Heart disease not present.  

For these reasons, software vendors are encouraged to store negation in the electronic health record rather than looking for a concept which encapsulates its meaning.  Storing negated findings in a separate field in the EHR, for example, would simplify querying data. 

(See also Appendix: Principles for Accepting Content in the International Release)

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