The results in the previous section clearly indicate that a large, comprehensive terminology coding system (SNOMED CT) provided a more complete coverage of desired terms than either of the two focused coding systems (ICF and ICNP). However there are important caveats to this statement. First, the ICF and ICNP are both classification systems, not formal terminologies. The coded terms within both systems reflect a high-level grouping of data, not an exhaustive list of all possible terms that could be classified into specific groups. Of note, both the ICF and ICNP were developed with intent to expand their content over time. Second, both the ICF and the ICNP classification systems provide textual definitions of each coded term. If the definitions within the ICF and ICNP were formally expressed and coded, a more extensive content coverage would have been evident. SNOMED CT does not provide textual definitions of terms. The meaning of terms must be inferred based on the placement of the term within the SNOMED CT hierarchies. This presents an obvious source of potential error in any content coverage study.
As yet, there is no recommendation from the NCVHS on the use of single, comprehensive health care terminology such as SNOMED CT or on an alternative federated approach with many different systems providing terms. Should the NCVHS recommend a single comprehensive reference terminology (for example, SNOMED CT), classification systems such as the ICF and ICNP could be rolled into such a terminology system in ways that retain the intent and focus of those classification systems. This will require significant development work for both the ICF and the ICNP in order to formally express their structure in a manner that is compatible with the reference terminology system. Alternatively, should the NCVHS recommend a federated approach whereby multiple large and small scale systems are used, similar development work will be required in order to integrate and assure interoperability of various coding systems within and across electronic health information systems.