Plasma C-Peptide Assay	

Plasma C-peptide levels, like those of insulin, rise after B-cell stimulation but, because of its larger pool size and slower turnover rate, large fluctuations in plasma C-peptide are less common. Overnight fasting peripheral venous plasma C-peptide levels generally fall within the range 500–900pmol/l but, like plasma insulin levels, are suppressed by prolonged fasting, vigorous exercise, and hypoglycaemia. Except in the presence of endogenous hyperinsulinism, sulphonylurea poisoning and some forms of autoimmune hypoglycaemia, plasma C-peptide concentrations are invariably suppressed to below 300pmol/l by hypoglycaemia (Fig. 20.17). This is the basis of the insulin hypoglycaemia test for endogenous hyperinsulinism due to insulinoma.