Laboratory Tests for Diagnosing Insulinoma
The diagnosis of insulinoma requires a supervised 48-72 hour fast with measurement of glucose, insulin, C-peptide, and proinsulin at the time of hypoglycemia, using specific diagnostic thresholds to confirm inappropriate insulin secretion. 1
Gold Standard Diagnostic Test
The supervised prolonged fast is the first-line diagnostic test for insulinoma and should be performed until hypoglycemia occurs or the 48-72 hour time limit is reached. 1 This test requires hospitalization due to the risk of severe hypoglycemic episodes but remains the most reliable diagnostic approach. 2
Laboratory Criteria at Time of Hypoglycemia
When hypoglycemia develops during the supervised fast (blood glucose <40-45 mg/dL), the following laboratory values confirm the diagnosis of insulinoma: 1
- Insulin level >3 mcIU/mL (inappropriately elevated for the degree of hypoglycemia) 1
- C-peptide ≥0.6 ng/mL (confirms endogenous insulin production) 1
- Proinsulin levels ≥5 pmol/L (elevated proinsulin-to-insulin ratio strongly suggests islet cell tumor) 3, 1
- Insulin-to-glucose ratio ≥0.3 (demonstrates inappropriate insulin secretion relative to glucose) 1
Critical Role of Each Laboratory Test
Insulin measurement demonstrates inappropriately increased plasma insulin concentrations in the face of low glucose, which is the hallmark of insulinoma. 3 The persistence of insulin secretion despite hypoglycemia distinguishes insulinoma from other causes of hypoglycemia. 3
C-peptide measurement is essential because it differentiates endogenous hyperinsulinemia (insulinoma) from surreptitious exogenous insulin administration. 3, 1 Elevated C-peptide levels (≥0.6 ng/mL) confirm that the insulin is being produced by the patient's own pancreatic tissue rather than being injected. 3
Proinsulin measurement provides additional diagnostic specificity, as an increased ratio of fasting proinsulin to insulin strongly suggests the presence of an islet cell tumor. 3 Some rare insulinomas secrete predominantly proinsulin rather than processed insulin, and these tumors would go undetected if only insulin levels were measured using modern specific immunometric assays. 4 Proinsulin assay is particularly helpful when immunoreactive insulin measured by specific new methods appears normal. 4
Important Caveats and Pitfalls
Avoid measuring sulfonylurea levels to exclude factitious hypoglycemia from oral hypoglycemic agents, as the absence of plasma sulfonylurea combined with elevated insulin and C-peptide confirms endogenous hyperinsulinism. 5
Be aware that modern insulin assays have excellent specificity with no cross-reactivity with intact or Des 31,32 proinsulin, but this means rare proinsulin-secreting tumors require specific proinsulin measurement for diagnosis. 4
The absence of inappropriately elevated glucose, insulin, and proinsulin concentrations during fasting hypoglycemia makes the diagnosis of an islet cell tumor most unlikely, and alternative explanations should be sought. 3
Proton pump inhibitors can cause spuriously elevated chromogranin A levels, which may complicate the diagnostic workup if neuroendocrine tumor markers are being assessed. 1
Alternative Testing Considerations
While a 5-hour oral glucose tolerance test with insulin-to-glucose and C-peptide-to-glucose ratios has been studied as an outpatient alternative, this approach is not recommended as first-line testing. 6 The supervised prolonged fast remains the gold standard because insulinomas typically cause fasting rather than postprandial hypoglycemia, and some insulinomas may be glucose-responsive, making provocative testing unreliable. 7