The optimized algorithm (ratio of type 1 to type 2 codes ≥ 4, and at least 1 insulin prescription within 90 days) performed highly accurately at ages at diagnosis < 20 years, but it also generated close estimates of the proportion of T1D among adults, as misclassified T1D and T2D cases were approximately balanced. This evidence concerns the gene INS and type 1 diabetes mellitus.