Elecsys Insulin Assay: Free Insulin Determination and the Absence of Cross-Reactivity with Insulin Lispro

Abstract

Insulin assays are used in the identification of rare hypoglycemic syndromes, and may be useful in classifying the different types of diabetes (1)(2)(3). Hemolysis (4)(5)(6) and circulating anti-insulin antibodies (7)(8) are sources of interference in insulin determinations. Anti-insulin antibodies may be present in the serum of insulin-treated diabetic patients, even when they are treated with biosynthetic human insulin, and may also be found in the serum of type 1 diabetic patients before insulin administration. These antibodies interfere in competitive and noncompetitive insulin immunoassays. In the increasingly used two-site noncompetitive assays, anti-insulin antibodies may yield increased results. Thus, anti-insulin antibodies must be eliminated, usually through precipitation with polyethylene glycol, before the concentration of free (active) insulin is determined (9). In addition to these considerations, both biosynthetic human insulin and insulin lispro, a fully potent analog with a faster absorption rate, are used therapeutically so that not only human endogenous and exogenous insulin, but also insulin lispro may be present in samples in which the insulin assay is to be used. In the present study we performed a preliminary evaluation of an insulin electrochemiluminescence immunoassay recently developed by Roche Diagnostics (Mannheim, Germany) for the Elecsys® analyzer. We studied the analytical performance of this new insulin assay and assessed the results of insulin measurements in serum samples with and without anti-insulin antibodies. The Elecsys assay uses an 18-min incubation time and a 20-μL sample volume, and has a stated dynamic range of 0.2–1000 mIU/L (1.39–6945 pmol/L). Assays were performed on the Elecsys 2010 analyzer in singleton, according to the manufacturer’s instructions, with two different lot numbers. The Elecsys results were compared with those of our routine method (Bi-Insulin IRMA; Bio-Rad) in EDTA-plasma samples from 110 diabetic patients treated with oral hypoglycemic drugs …