Using SNOMED CT^®as a reference terminology to cross map two highly pre-coordinated classification systems

We hypothesized that SNOMED CT, a granular formal reference terminology, could be used to assist in the creation of a valid crosswalk between two administrative classifications: the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) and the U.S. Veterans Benefits Administration (VBA) disability code set. To establish a baseline, we created an ICD-9-CM terminology server and directly mapped textual descriptions of the VBA disability codes to ICD-9-CM. We next mapped ICD-9-CM and the VBA Disability codes to SNOMED CT. The SNOMED CT mappings were matched across classification systems and terms from related concepts were displayed for an expert coder's review. We report the rate of direct ICD-9-CM to VBA Disability Code mapping (26%), the eventual success of the SNOMED CT based crosswalk (95%) and the rate at which the reviewer had to add codes to complete the mapping (99%). The method using the SNOMED CT crosswalk provided significantly better coverage than the ICD-9-CM direct mapping alone (Pearson Chi Square test; p<0.001). We conclude that SNOMED CT can be a useful adjunct to direct mapping between administrative classifications.