What is true for Insulinoma: Elevated C-peptide (C-peptide) level, Low blood sugar (Hypoglycemia), or Elevated serum insulin level?

Diagnostic Criteria for Insulinoma

The correct answer is C. All of the choices are correct. Insulinoma is characterized by elevated C-peptide levels, low blood sugar (hypoglycemia), and elevated serum insulin levels. 1, 2

Key Diagnostic Features of Insulinoma

• Insulinoma presents with hypoglycemia (blood glucose <40-45 mg/dL), which is a hallmark clinical feature of this condition 1, 2
• Elevated serum insulin levels >3 mcIU/mL (usually >6 mcIU/mL) during hypoglycemia are diagnostic of insulinoma, reflecting inappropriate insulin secretion 1, 2
• Elevated C-peptide levels (≥0.6 ng/mL) are consistently present in insulinoma and help differentiate endogenous hyperinsulinemia from exogenous insulin administration 2, 3
• An insulin-to-glucose ratio of 0.3 or greater during hypoglycemia further confirms the diagnosis 1, 2

Diagnostic Approach

• The supervised 48-72 hour fast is the gold standard test for diagnosing insulinoma, continued until hypoglycemia occurs or the time limit is reached 2
• At the termination of the fast, the diagnostic triad includes:
  • Blood glucose <40-45 mg/dL
  • Insulin level >3 mcIU/mL (usually >6 mcIU/mL)
  • C-peptide ≥0.6 ng/mL 2, 3
• Testing for urinary sulfonylurea helps rule out factitious hypoglycemia 1

Clinical Presentation

• Patients typically present with neuroglycopenic symptoms (confusion, lethargy, seizures) 2
• Symptoms often occur during fasting, particularly in the morning 2
• Weight gain may occur due to frequent eating to avoid hypoglycemia 2

Diagnostic Pitfalls to Avoid

• C-peptide levels are crucial for differentiating between endogenous hyperinsulinemia (insulinoma) and exogenous insulin administration (factitious hypoglycemia) 2, 4
• Recent research indicates that C-peptide levels <9.6 ng/ml have 97.3% sensitivity for insulinoma diagnosis 4
• Insulin levels <75 μU/ml have 93.4% sensitivity for insulinoma diagnosis 4
• Autoimmune hypoglycemia typically presents with significantly higher insulin and C-peptide levels compared to insulinoma 4

Imaging for Localization

• CT or MRI scans should be performed to rule out metastatic disease, with sensitivities of 82-86% 1, 5, 4
• Endoscopic ultrasound (EUS) has the highest sensitivity (82-93%) for detecting small pancreatic tumors 5, 4
• Insulinomas are less consistently octreotide-avid than other pancreatic neuroendocrine tumors, making Octreoscan less useful 1