Diagnosis of insulinoma using the ratios of serum concentrations of insulin and C-peptide to glucose during a 5-hour oral glucose tolerance test.

The 72-hour fast test is the current standard for the diagnosis of insulinoma. However, to conduct this test patients require hospitalization due to the chance of severe hypoglycemic episodes. Thus, it is costly and stressful for the patient. An out-patient test would serve the patient better and be more economical. Our aim was to evaluate the value of insulin to glucose and C-peptide to glucose ratios during a prolonged 5-hour oral glucose tolerance test (5-hour OGTT) in qualitative diagnosis of insulinoma, and to identify the optimal threshold for clinical screening. Initially, 15 subjects with pathological insulinoma and 12 control subjects with reactive hypoglycemia were enrolled in the study. A further 75 subjects with symptoms of hypoglycemia as a chief complaint at their initial clinic visit were subsequently screened. Serum insulin, C- peptide levels and blood glucose were quantified after a 5-hour OGTT in all participants and the ratios of serum concentrations of insulin and C-peptide to glucose were calculated. Subjects with insulinoma had significantly different insulin-to-glucose and C-peptide-to-glucose ratios from reactive hypoglycemia at the times of fasting, 4-hour post glucose load and 5-hour post glucose load. Higher specificity (73.08%) and sensitivity (82.67%) were achieved with the combined insulin-to-glucose ratio at the 5-hour post load and the C-peptide-to-glucose ratio at fasting. In combination, ratios of insulin and C-peptide release relative to blood glucose levels, measured during a 5-hour OGTT, may have important clinical value in the diagnosis of insulinoma.