Insulin antibody assay: a statistical evaluation of sensitivity, precision and reproducibility in healthy subjects

Eighty-seven sera from healthy non-diabetic subjects, negative for islet cell antibodies, were analysed for 125I-insulin serum binding capacity by a commercial radioimmunoassay. In order to define the detection limit of the assay for insulin antibodies we used the corrected binding capacity to 125I-insulin of the normal sera obtained by competitive inhibition with excess of unlabelled insulin. To test the reproducibility of the results as a function of the tracer decay the sera were re-examined after intervals of 17 and 37 days, when a drift of more than 1% was found in the period investigated. Using the corrected bindings improved reproducibility considerably. The extra variation between the three assays, taking into account the decay of the tracer, was negligible. The main component of error contributing to these variations among individuals was found to be the precision, defined as the mean standard deviation among the replicates, which is SD(c) = 0.325%. We conclude that this assay is suitable for the screening of positive insulin autoantibodies in pre-diabetic subjects, and recommend that the estimate of precision from the replicates with excess insulin be used and that the detection limit be set at three times the estimated SD(c).