ASSESSMENT OF IMMUNOASSAYS FOR INSULIN IN DIAGNOSTIC-TESTS FOR INSULINOMA

An immunoassay for insulin using a monoclonal insulin-specific antibod y has been marketed in Japan for 2 years. This new assay may yield dif ferent plasma insulin (IRI) values from those obtained by the conventi onal immunoassay because of its high specificity. In this study, we co mpared IRI values obtained by three different immunoassays, assays P, M1 and M2, the last two of which involved monoclonal antibodies. For n ormal subjects, similar IRI values were obtained from all three assays . In patients with insulinoma, IRI values from assay MI were 7-63% of those from assay P. Assays M1 and M2 gave similar values. Proinsulin i nterfered with assay P but not with assays M1 or M2. Differences betwe en the IRI(P) value and the IRI(MI) or IRI(M2) value were correlated w ith the concentration of proinsulin (r = 0.98). The IRI(P) value appea rs therefore to represent the sum of levels of insulin and proinsulin. We conclude, therefore, that an immunoassay with broad specificity (i .e. assay P) is better for screening for hyper-(pro)insulinemia. Immun oassays with narrow specificity (i.e. assays M1 and M2) may have benef its in some tests aimed at tumor localization.