Clinical and laboratory evaluation of a new specific ELISA for intact proinsulin

Intact proinsulin is a marker for pancreatic beta-islet cell secretion status, and elevated levels indicate insulin resistance of type 2 diabetic patients with a high diagnostic specificity. Determination of intact proinsulin in fasting morning plasma samples can be used to establish and/or identify the optimal anti-diabetic treatment as well as to monitor a potential protective treatment effect on beta-islet cell dysfunction. For widespread use as a routine marker, simple and reliable assays must become available for measurement in clinical laboratories. This study was performed to evaluate a new ELISA method specific for intact proinsulin determination compared with a commercial chemiluminescence test. Intra-assay imprecision was determined to be 2.4 - 5.5% (inter-assay: 3.2 - 4.3%). A sample cohort derived from non-diabetic healthy subjects (66 female, 29 male, age (median, range): 37 (19-77) years) was used to calculate a reference range for non-diabetic individuals of 1.8 - 11.0 pmol/l. The comparison of samples obtained from 7 diabetic patients (2 female, 5 male, age: 66 (44-72) years) from oral glucose tolerance tests resulted in an excellent correlation between the ELISA and the chemiluminescence assay (r = 0.989; p < 0.001). A sample stability investigation with different sample specimens revealed that intact proinsulin is stable for two days at room temperature in EDTA whole blood and heparin whole blood tubes without centrifugation, while serum samples should be centrifuged immediately and frozen to retain intact proinsulin concentrations. As EDTA whole blood is the routine sample specimen for determination of HbA1c, a marker frequently measured in diabetic patients, this finding underlines the practicability of analyzing intact proinsulin from the same blood specimen. In conclusion our study revealed that the new ELISA shows excellent agreement with the commonly used chemiluminescence immunoassay. The ELISA can easily be introduced into routine laboratories and does not require any further specific instrumentation. Our additional finding that intact proinsulin is stable in EDTA whole blood samples, which can be obtained from the routine sample for HbA1c measurement, is increasing the probability of the acceptance of this marker for routine assessment of R-cell dysfunction and insulin resistance in type 2 diabetes.