What do identifiers in HL7 identify? An essay in the ontology of identity

Health Level 7 (HL7) is an organization seeking to provide universal standards for the exchange of healthcare information. In a document entitled ‘HL7 Version 3 Standard: Data Types’, the HL7 organization advances descriptions of data types recom- mended for use as identifiers. We will argue that the descriptions supplied provide insufficient guidance as to what exactly the entities are which these data types uniquely identify. Are they real things, such as persons or pieces of equipment? Or are they representations of such real things in information artifacts? We here outline the problems faced by HL7 in providing answers to such questions, problems which arise because of the lack of anything like a coherent ontology in the HL7 standard, and we make some recommendations for future improvements.