Subsets of SNOMED CT concepts can be used to categorize patient data by testing for membership in a predefined subset. The diagram below illustrates the use of a simple reference set which includes references to rare disease concepts. SNOMED CT does not currently have a defined mechanism to distinguish rare diseases, so this simple reference set is defined extensionally (i.e. by enumeration). This simple reference set is used to create a cohort, by categorizing patients according to their SNOMED CT encoded records, and specific values of contextual metadata. By comparing the patient diagnosis with the concepts included in the reference set it is possible to count the total number of patients who suffer from rare diseases.
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