Self‐collection by Diabetic Patients of Capillary Blood for Free Insulin Monitoring; Reduction by Diamide of Haemolysis‐induced Insulin Loss

Abstract

The need to precipitate bound insulin immediately after withdrawal of blood and the tendency to haemolysis, which reduces immunoassayable insulin, have prevented development of methods of self-collection of capillary blood for later free insulin measurement. We therefore investigated the use of the thiol-oxidizing agent, diamide, to prevent insulin loss with haemolysis and developed a self-collection procedure with capillary tubes pre-filled with diamide and polyethyleneglycol (PEG, for separation of free and bound insulin). Diamide (final concentration 5 mmol l-1) reduced serum insulin loss from 48 +/- 4 (+/- SE) to 11 +/- 4% (p less than 0.001) in maximally-haemolysed samples. The effect of diamide was concentration-dependent up to 5 mmol l-1. Diamide had no effect on the standard curve for radioimmunoassay of insulin. Levels of serum free insulin in self-collected capillary blood were significantly correlated with venous serum free insulin in 22 non-diabetic subjects (r = 0.92, p less than 0.001), 52 Type 1 diabetic patients (r = 0.86, p less than 0.001), and 18 Type 2 diabetic patients (r = 0.97, p less than 0.001). Mean capillary free insulin concentration was higher than in venous serum (22% in normal subjects, 64% in Type 1, and 23% in Type 2 diabetic patients). Storage at room temperature of capillary blood containing PEG/diamide for 72 h did not alter immunoassayable insulin concentrations.