Before mapping, there was general agreement between the two terminologists on mapping rules including how the quality of the mapping relationships would be defined (see below) and how post coordinated concepts would be represented.
If a pre-coordinated concept cannot be found, post-coordination will be attempted.
Whenever a pre-coordinated concept is available, that should be the first choice.
If a match is found and the concept is active, the term can be encoded with a pre-coordinated concept.
These were all to be mapped similarly using agreed upon post-coordinated concept groupings in SNOMED CT (e.g. Finding by auscultation [finding] Associated with [attribute]).
If the term cannot be matched using a pre-coordinated concept, an attempt will be made to represent the term with multiple concepts, or post-coordination.
If the pre-coordinated concept is a primitive concept, an alternative is to create a post-coordinated expression to ensure the necessary semantics are recorded.
If no pre-coordinated concept or post-coordinated expression can adequately represent a data item, and if there is no desire to create no extension, that term is marked as unencodeable.
Sequences of events could potentially be modeled in the i2b2 hive as a single, pre-coordinated concept, but would require the addition of such concepts to the ontology, as well as pre-processing of the data for identifying such concepts within the observations.
This shortcoming is especially important when importing data from structured clinical documentation into the i2b2 hive, since a consequence of this limitation is that every atomic observation will have to be available as a pre-coordinated concept, which is not practical in an environment with a large-scale EMR implementation.
Our experience with the process of coding with post-coordination and reuse of post-coordinated concepts in SNOMED CT is that the procedure is complicated and requires supportive tools if done by end users.
