BACKGROUND: Measurement of circulating insulin may improve the classification and management of diabetes mellitus and assist in treating people with insulin resistance. METHODS: A work group convened by the American Diabetes Association evaluated results for a panel of 39 single donor sera measured by 10 commercial insulin methods from 9 manufacturers against an isotope dilution-liquid chromatography/tandem mass spectrometry (IDMS) measurement procedure calibrated using purified recombinant insulin. We used a candidate primary (pure substance) reference material, pooled serum, and single donor sera to evaluate approaches to achieve improved agreement of results between the routine and reference measurement procedures. RESULTS: Four of 10 methods had ≥95% of individual serum results within 32% of the IDMS concentrations. However, the bias vs IDMS was more than 15.5% for 7 of 10 methods in 36%-100% of individual samples. A purified recombinant insulin preparation used as a common calibrator did not improve harmonization of results among routine methods but was not used as instructed by all participants. Calibration using serum pools achieved bias <15.5% for nearly all results in the concentration range covered by the pools (>60 pmol/L). Calibration using a panel of individual sera was the most effective to improve harmonization of results over the full measuring range. CONCLUSIONS: Agreement among methods can be improved by establishing traceability to the IDMS procedure using a panel of native sera. Pooled sera may be useful as trueness control materials. The usefulness of the pure insulin primary reference material [candidate reference material for insulin (cRMI)] requires clarification of protocols used by manufacturers.