Limitations of the Plasma C-Peptide Assay	

As ordinarily performed (Fig. 20.18) plasma C-peptide assay is neither very sensitive nor very specific; false-positive results can occur, especially in patients with factitious hypoglycaemia due to sulphonylurea abuse. False-negative responses can also occur because of suppression of C-peptide secretion in patients with insulinomas, possibly due to increases in circulating plasma adrenaline levels. Both the sensitivity and specificity of the insulin hypoglycaemic test can be improved by using an infusion of low doses of exogenous insulin under controlled conditions in order to maintain a constant and reduced blood sugar level (a hypoglycaemia clamp procedure), but this is expensive, tedious and has no advantage over simpler tests such as measurement of overnight or fasting insulin, C-peptide and proinsulin levels, and the vigorous exercise test.