COMPARISON OF THE DETERMINATION OF INSULIN BY A MONOCLONAL ANTIBODY-BASED IMMUNOFLUOROMETRIC ASSAY AND BY RADIOIMMUNOASSAY

This paper describes an immunofluorometric assay (IFMA) for insulin an d compares it with the classical radioimmunoassay (RIA). Monoclonal an tibodies against insulin were produced and used to develop the IFMA. O ne, immobilized on microtiter plates, was used for capture, the other, labelled with Europium, was used as tracer antibody. The IFMA present s sensitivity to an amount of insulin of 3 pmol/1 and acceptable value s for intra- and interassay error. The IFMA presented superimposable c urves for human insulin, Arg(65)/Gly(66)-split proinsulin and des-Lys( 64),Arg(65), and no cross-reactivity with human proinsulin, Arg(32)/Gl u(33)-split and des-Arg(31),Arg(32). The RIA showed 100% cross-reactiv ity with human proinsulin, 90% with Arg(32)/Glu(33)-split, 193% With A rg(65)/Gly(66)- split, 340% with des-Arg(31),Arg(32) and 170% with des -Lys(64),Arg(65). The assays were used to measure insulin in 300 serum samples from 50 subjects submitted to an oral glucose tolerance test (OGTT). Twenty were normal, 10 had impaired glucose tolerance and 20 n on-insulin-dependent diabetes mellitus. The mean value (+/- SEM) obtai ned by IFMA was 166.7 +/- 12.1 pmol/l and the mean value obtained by R TA was 339.6 +/- 18.6, with a correlation of r = 0.80 (P<0.01). Compar ison of basal insulin levels of the different groups of individuals us ing IFMA or RIA led to the same conclusions. The area under the curve showed statistically significant differences only for the comparison b etween normal lean subjects and individuals with impaired glucose tole rance, when measured by RIA. Our data stress the importance of methodo logy definition when comparing insulin results. The specificity obtain ed with monoclonal antibody-based immunometric assays, in addition to other intrinsic qualities, indicates that this can be the assay of cho ice for insulin measurement.