The interference of insulin antibodies in insulin immunometric assays

We investigated the interference of insulin antibodies in two insulin immunometric assays (BioRad and Elecsys) by measuring direct and free insulin in plasma from 30 patients without insulin antibodies (group 1), as screened by a sensitive radiobinding assay, and in plasma from 80 patients with insulin antibodies (group 2). In group 1, the direct/free insulin ratio did not differ from 1, showing the equivalence of free and direct insulin results in theses samples. In group 2, this ratio was markedly increased (mean: BioRad 2.63, Elecsys 5.02) and correlated positively with the insulin antibody radiobinding assay result (r=0.92 for the correlation between BioRad and Elecsys assays after logtransformation of the ratios). In samples containing insulin antibodies, direct insulin concentration was frequently lower than total (bound and unbound) insulin measured with the BioRad and Elecsys assays. This study underlines the interference of insulin antibodies in insulin immunometric assays and the importance of assessing an insulin immunometric assay for sensitivity towards the presence of these antibodies.