Diagnosis depends upon the demonstration of symptomatic fasting hypoglycaemia in the presence of inappropriately high plasma insulin and C-peptide concentrations. This combination is almost pathognomonic of endogenous hyperinsulinism due to insulinoma, functional hyperinsulinism of infants (nesidioblastosis), or sulphonylurea poisoning.
Several factors determine the plasma insulin level in the blood, one of the most important being the arterial blood glucose concentration.
Large and spontaneous fluctuations in peripheral plasma insulin concentrations often occur in patients with insulinomas.
