Clinical and laboratory evaluation of specific chemiluminescence assays for intact and total proinsulin.

Measurement of proinsulin is an important tool in the assessment of pancreatic beta cell function in patients with type 2 diabetes. The goal of this study was to perform a technical and clinical evaluation of two specific chemiluminescence assays (CLIA) for the determination of intact and total proinsulin in comparison to a radioimmunoassay (RIA) method for the measurement of total proinsulin. A total of 191 serum samples from patients with type 2 diabetes were used to perform a regression analysis. The total proinsulin CLIA showed higher proinsulin levels than the two other proinsulin assays (mean +/- SD: 55.9 +/- 58.1 pmol/l, p < 0.001 in both cases). The intact proinsulin CLIA (22.5 +/- 20.9 pmol/l) gave lower values than the RIA for total proinsulin (31.9 +/- 25.4 pmol/l, p < 0.001 vs. CLIA, r = 0.948). The RIA has a 95% cross-reactivity to des31,32-proinsulin, which is secreted during the process of beta cell deterioration. The intact proinsulin CLIA has virtually no cross-reactivity with des31,32-proinsulin (1.4%) and is therefore more specific for intact proinsulin than the RIA. This test does not measure further degradation products, in contrast to the total proinsulin CLIA. The CLIA is, therefore, more specific for total proinsulin measurement than the RIA. Both CLIAs could be performed much faster (4 hours) than the RIA method (75 hours/ 4 days). In conclusion, the CLIA methods show improved qualitative outcomes, higher specificity and several technical advantages over the RIA method.