Competing interpretations of disorder codes in SNOMED CT and ICD.

Under ontological scrutiny we have identified two competing interpretations of disorder concepts in SNOMED. Should codes be interpreted as representing pathological conditions themselves or the situations in which a patient has those conditions? This difference has significant implications for the proposed harmonization between SNOMED CT and the new ICD-11 disease classification and indeed for any systematic review of the correctness of the SNOMED CT hierarchies. Conditions themselves are distinct, whereas in any given situation a patient may have more than one condition. In such cases, SNOMED codes that represent combinations of conditions - which can be regarded as "additive" - are evidence for interpreting the codes as referring to situations. There are clearly some such codes. We conducted a survey to determine the extent of this phenomenon. Three criteria were used - analysis of the SNOMED CT fully specified name, the corresponding logical definition, and the children of the concept under scrutiny. All three showed that at least 11% of concepts met our criteria for representing situations rather than conditions, with a satisfactory inter-rater reliability for the first two. We, therefore, conclude that if a uniform interpretation of SNOMED disorder codes is desired, they should be interpreted as representing situations.