ATC-to-RxNorm mappings - A comparison between OHDSI Standardized Vocabularies and UMLS Metathesaurus

The World Health Organization global standard for representing drug data is the Anatomical Therapeutic Chemical (ATC) classification. However, it does not represent ingredients and other drug properties required by clinical decision support systems. A mapping to a terminology system that contains this information, like RxNorm, may help fill this gap. This work evaluates and compares the completeness of mappings from the chemical substance level (5th-level) ATC classes to RxNorm ingredient concepts in the OHDSI Standardized Vocabularies (OSV) and the Unified Medical Language System (UMLS) Metathesaurus.

To check the concordance between OSV and UMLS we compared the included contents of ATC and RxNorm not only in OSV and UMLS but also in BioPortal and the National Library of Medicine (NLM) repository. For each repository, we determined the number of 5th-level ATC concepts, RxNorm ingredient concepts, missing classes and concepts, and the ATC categories with the most missing concepts. The mappings from ATC to RxNorm in OSV and UMLS were compared, and we determined the number of mappings in common, and the mapping differences, which we categorized. We applied the mappings from OSV and UMLS on a sample of Electronic Health Record (EHR) data.

NLM contained the most ATC and RxNorm concepts. UMLS contained more missing mappings (null mappings) than OSV, 1949 versus 916. Most mapping differences were in the "unknown ingredient in the ATC label" category, for which UMLS provided no mappings. UMLS had a higher coverage of mappings in the sample EHR data than OSV, 96.5% versus 91%.

In conclusion, opting for OSV rather than UMLS is generally preferable for an ATC to RxNorm mapping since OSV provides more mappings. However, the results of the sample data show that UMLS can have fewer null mappings in concrete applications.