A high-quality enzyme immunoassay for specific quantification of human insulin in serum or plasma. The Mercodia insulin assays have little or no cross-reactivity to insulin analogs, rat or mouse insulin, which enables distinguishing endogenous human insulin, both in patients undergoing treatment with insulin analogs, and in rat or mouse models. The Mercodia Insulin ELISAs are the only ELISA assays to satisfy the total error requirements in the Insulin Standardization Program.