Question: Why Postcoordinate?
Traditional terminologies, such as ICD, are enumerative: they provide a priori a fixed list of all the legitimate things that can be recorded using them, and assign each a code. If no code exists for what a user wants to say, they can not say it. They may ask for the code to be added to the fixed list, but the time between releases of the codeset where such feature requests could be included is often very long.
Despite their limitations, enumerated terminologies and classifications continue to serve medicine reasonably well in the roles for which they were designed, typically recording single codes to summarise entire patient episodes, or causes of death. They are, however, insufficiently expressive to adequately record much detail about the day-to-day condition of the patient or the process of caring for them. As a consequence, such schemes can not support the kind of clinical record needed to underpin decision support.
Compositional terminologies, such as the OpenGALEN Common Reference Model or SNOMED CT, have been engineered using new technology to address this problem of expressivity: like traditional terminologies, out-of-the box they offer a set of pre-enumerated codes covering only a small fraction of all the things that might be said. But when a user cant find what they want, they can create (postcoordinate) it on demand, defining what they need in terms of the pre-enumerated codes.
An obvious example of where SNOMED CT content does not currently contain a priori all useful clinical concepts is laterality: there are only a very few pre-enumerated codes for either left- and right- sided body structures, or for surgical procedures on them. Were all reasonable permutations of lateralised anatomy or procedure to be added as pre-enumerated concepts, the number of SNOMED codes would increase by approximately 150,000.
Another example is the priority of a procedure: was it elective, routine, emergency, rescheduled etc? SNOMED recognises 15 different possible types of priority, under the concept Priorities. Pre-enumerating all possible permutations of a Procedure plus a priority would add a further 780,000 new SNOMED Concepts. The combination of simultaneously pre-enumerating all possible permutations of both laterality and priority, however, would require an additional 2.4 million new SNOMED concepts.