The Common Terminology Services (CTS) specification was developed as an alternative to a common data structure. We have published the HL7 CTS specification and a reference implementation of the specification.

The HL7 Common Terminology Services (HL7 CTS) is an Application Programming Interface (API) specification that is intended to describe the basic functionality that will be needed by HL7 Version 3 software implementations to query and access terminological content. It is specified as an API rather than a set of data structures to enable a wide variety of terminological content to be integrated within the HL7 Version 3 messaging framework without the need for significant migration or rewrite.

Learn about this at http://informatics.mayo.edu/LexGrid/index.php?page=ctsspec