This research paper examines the prevalence of rare diseases in a U.S. commercial population using claims data. ICD-10 diagnosis codes associated with rare disease were sourced from Orphanet, a prominent worldwide repository for rare disease data. In the analysis, 19% of the commercially-insured population was observed to have at least one ICD-10 diagnosis code associated with a rare disease, which suggests that many ICD-10 diagnosis codes for rare diseases are too broadly defined. This report discusses the benefits of increased specificity in ICD-10 codes and offers suggestions to enable more accurate identification of rare diseases in claims data.