[Hypoglycaemia with low insulin levels: what insulin tests do not measure]

Insulin can be measured by immunochemical methods using polyclonal or monoclonal antibodies. Monoclonal antibodies are specific in the detection of pure human insulin, and may show little to no cross reactivity with pro-insulin or recombinant insulin. Polyclonal antibodies, however, do show such cross reactivity. Most medical laboratories use commercial (monoclonal) methods to measure insulin 75% of which are not capable of detecting pro-insulin or exogenous insulin. This pitfall in diagnostics may lead to prolonged uncertainty for both patient and physician, which we illustrate with two patients. The first patient was a 45-year-old woman with DM type 1 who for years suffered from hypoglycaemic attacks. Factitious hypoglycaemia went undiagnosed because our monoclonal assay did not detect the overdose insulin analogues. The second patient was a 47-year-old woman with recurrent hypoglycaemic attacks. An insulinoma, which produced pro-insulin, was only detected after using polyclonal insulin and specific pro-insulin assays.