Effects of two commercial insulin immunoassays on insulin c-peptide correlation

The molar ratio of insulin to C-peptide in peripheral venous blood argues for surreptitious/inadvertent insulin administration or insulinoma/sulfonylurea ingestion as the cause of the hypoglycemia. Commercial immunoassays for insulin are prone to discrepancy because of heterogeneity in circulating forms that react with antisera and cross-reactivity with proinsulin. In this study we compared molar ratio of insulin to C-peptide analyzed by different assays in independent patient groups. In this retrospective study patients admitted to Gaziantep University Hospital Central Laboratory between 2010-2011 recruited the study group. Patients with unknown or positive history of insulin therapy were excluded. Study groups were constructed according to insulin analysis methods; 78 patients whose insulin levels have been assayed by Abbott Architech i2000 (group 1) and by Immulite2000 (n:132, group 2). C peptide levels of the both groups have been analyzed by Immulite2000. Insulin to C-peptide ratio is higher for groupl than group 2 (p>0.05). The correlation coefficient of group 1 was lower than group 2 (r: 0,46 vs. r: 0,65, p<0.001). According to our study insulin C-peptide correlation coefficient was quite low when insulin and C peptide were analyzed with different commercial assays. Although both insulin tests are calibrated against reference preparation, different immunoreactivity of the reagent antibodies to immuncomplexes, heterophile antibodies can lead to such a discrepancy. Results of the study emphasize the importance of standardization for commercial assays insulin assays.