A 32-year-old woman presents with recurrent episodes of severe hypoglycemia (blood glucose 35–45 mg/dL) occurring 2–3 hours after meals, accompanied by palpitations and diaphoresis. She denies any history of diabetes or medication use. Physical examination is unremarkable. Fasting blood glucose is 92 mg/dL. Which investigation is most appropriate to confirm the suspected diagnosis of insulinoma?

Explanation

## Diagnosis of Insulinoma: The Whipple Triad and Confirmatory Testing ### Clinical Context The patient presents with **Whipple's triad**: (1) symptoms of hypoglycemia (palpitations, diaphoresis), (2) documented low blood glucose (35–45 mg/dL), and (3) relief with glucose administration. The timing (postprandial) and normal fasting glucose suggest an insulin-secreting tumor (insulinoma). ### Gold Standard Investigation **Key Point:** Serum insulin and C-peptide levels measured **simultaneously during documented hypoglycemia** are the diagnostic gold standard for insulinoma. **Rationale:** - In normal physiology, hypoglycemia suppresses insulin secretion (insulin < 3 mIU/L, C-peptide < 0.6 ng/mL). - In insulinoma, insulin and C-peptide remain **inappropriately elevated** despite hypoglycemia (insulin > 3 mIU/L, C-peptide > 0.6 ng/mL). - This **paradoxical elevation** of insulin in the face of low glucose is pathognomonic. - C-peptide measurement confirms **endogenous insulin secretion** (excludes factitious hypoglycemia from exogenous insulin injection). ### Why Simultaneous Measurement During Hypoglycemia? 1. **Timing is critical**: Insulin must be measured when glucose is genuinely low (< 55 mg/dL), not during euglycemia. 2. **Fasting insulin alone is insufficient**: A single fasting level may be normal in some insulinoma patients, especially if the tumor secretes episodically. 3. **C-peptide distinguishes endogenous from exogenous insulin**: Exogenous insulin injection would show high insulin but suppressed C-peptide. ### Diagnostic Criteria | Parameter | Normal (Hypoglycemia) | Insulinoma | |-----------|----------------------|------------| | **Serum Glucose** | < 55 mg/dL | < 55 mg/dL | | **Serum Insulin** | < 3 mIU/L | > 3 mIU/L (often > 10) | | **C-Peptide** | < 0.6 ng/mL | > 0.6 ng/mL | | **Proinsulin** | < 20% of insulin | Often elevated | **High-Yield:** The **insulin-to-glucose ratio** (mIU/L ÷ mg/dL × 100) > 0.3 during hypoglycemia is also highly suggestive of insulinoma. ### Localization Studies (After Biochemical Confirmation) Once diagnosis is biochemically confirmed, imaging (CT, MRI, endoscopic ultrasound) is used to localize the tumor. ### Clinical Pearl Many insulinomas are small (< 2 cm) and benign. Biochemical confirmation precedes imaging because imaging may miss small tumors, but biochemistry is never wrong if done correctly during hypoglycemia.