Lee CK, Ha HJ, Oh SJ, et al. - Via a nationwide validation study of diagnostic algorithms, experts recognized cases of inflammatory bowel disease (IBD) within the Korea National Health Insurance System (NHIS) database. A random nationwide sample of 1,697 individuals (848 potential cases and 849 negative control cases) from 17 hospitals were involved for validation. Outstanding performance with the lowest error rate were attained by a combination of the ICD-10 code, ≥ 1 claims for healthcare encounters, and ≥ 1 prescription claims (reference algorithm). The multiple sampling test validated that the reference algorithm attains the best performance relative to IBD diagnosis. Poorer performance in comparison with that of the reference algorithm, especially for the diagnosis of individuals associated with secondary hospitals was shown by algorithms involving the Rare and Intractable Diseases registration codes. No statistical variation depending on the hospital volume or IBD type were exhibited by the performance of the reference algorithm. Hence, the reference algorithm as a uniform standard operational definition for prospective studies using the NHIS database was strongly suggestive.