To what entities does an ICD-9-CM code refer? A realist approach

I take a view of ICD-9-CM codes as diagnostic statements, where these statements are about entities that exist in reality. I then represent these entities according to a realist view of disease, disorder, and diagnosis as defined by the Ontology for General Medical Science and using Referent Tracking templates. I illustrate the approach using ICD-9-CM codes that refer to systemic arterial hypertension. The approach is general and could help enable interoperability of data encoded with realist ontologies and data encoded with terminologies and administrative classifications, such as data from electronic health records and insurance claims.