Background: Circulating insulin concentration in serum or plasma provides important information for the estimation of insulin secretion and insulin resistance. Currently, lack of standardization of insulin assays hinders efforts to achieve consistent measures for treatment guidelines. Methods: A Workgroup convened by the American Diabetes Association evaluated 12 different commercial insulin methods from 9 manufacturers. Results: The within-assay CVs ranged from 3.7% to 39.0%, with 7 of 10 assays having a CV ≤10.6%. The among-assay CVs ranged from 12% to 66%, with a median value of 24%. A common insulin reference preparation did not change the among-assay CV and failed to improve harmonization of results among assays. Results from 6 of 10 assays agreed within the total error of 32% that is allowable based on biological variability criteria. Seven of 10 assays recovered insulin added to a serum pool within 15.5% of the expected concentration. In 9 of 10 methods, there was <2% cross-reactivity with intact human proinsulin, and 8 of 10 methods had <3% cross-reactivity with split (32, 33) proinsulin. For 9 of 10 assays, the cross-reactivity of des (64, 65) proinsulin exceeded 40%. Overall, most assays had acceptable imprecision and specificity for insulin, Conclusion: The discordance in test results for commercial insulin reagent sets is likely multifactorial and will require a continuing effort to understand the differences and achieve the desired consistency and harmonization among commercial immunoassays.