Plasma and Serum Insulin Determination by Enzyme Immunoassay

Abstract

Circulating plasma insulin is usually determined by radioimmunoassay (RIA) and very recently, in some institutes, it has also been determined by radioreceptor assay. Isotopically labeled insulin is necessary for both methods, so that a specially controlled laboratory is required for routine insulin determinations. Recently, a technique has been developed for coupling enzyme to plasma proteins including antibody.
This paper describes a study on the determination of plasma and serum insulin by enzyme immunoassay (EIA) using anti-insulin antibody labeled with horseradish peroxidase. The values obtained were compared with those obtained by RIA.
The intra- and interassay variances of the EIA method were 4.2% and 7.2% for plasma containing insulin at 31μU/m/, and 3.2% and 4.1% for plasma with 85μU/m/, respectively. Dilution tests using plasma with a very low concentration of insulin revealed a straight line starting from the 0 point. There were no differences between the values obtained for plasma and serum samples. Recovery tests also gave satisfactory results: the mean recovery was 99.5%.
A 100 g oral glucose tolerance test was performed in 60 subjects. Plasma samples obtained at 0 time and at 30, 60, 90. 120 and 180 min after glucose load were determined for insulin concentration by both EIA and RIA. The regression line and correlation coefficient between EIA (y) and RIA (x) were calculated as y=0.918x+ 3.60, and r=0.955, respectively.
This EIA method is simple and reliable, and does not require any specially-controlled room such as an isotope laboratory. The method is thus expected to have wide use in clinical medicine in the near future.