In a patient with suspected endogenous hyperinsulinemic hypoglycemia, what is the first‑line diagnostic study and how should a mixed‑meal tolerance test (MMTT) be performed if the 72‑hour fast is negative?

Diagnostic Approach to Suspected Endogenous Hyperinsulinemic Hypoglycemia

The 72-hour supervised fast is the first-line diagnostic test for suspected endogenous hyperinsulinemic hypoglycemia, though in practice 94.5% of insulinomas are diagnosed within 48 hours, making a 48-hour fast the practical standard. 1

When to Use the 72-Hour (48-Hour) Fast

The supervised fast is indicated specifically for patients presenting with fasting or neuroglycopenic symptoms that occur independent of meals—this pattern strongly suggests insulinoma rather than postprandial hyperinsulinemic conditions. 2

Key Diagnostic Criteria During the Fast

• Terminate the fast when plasma glucose falls below 55 mg/dL (3.0 mmol/L) with concurrent symptoms of hypoglycemia 3
• Simultaneously measure: plasma glucose, insulin, C-peptide, proinsulin, and beta-hydroxybutyrate 3
• Obtain urine/plasma sulfonylurea screen to exclude factitious hypoglycemia 2
• Diagnostic biochemical findings for insulinoma include: insulin >3 mcIU/mL (often >6 mcIU/mL) when glucose <40-45 mg/dL, insulin-to-glucose ratio ≥0.3, and elevated C-peptide ≥0.6 ng/mL (≈200 pmol/L) 4

Practical Timeline

• 42.5% of insulinomas manifest diagnostic hypoglycemia by 12 hours 1
• 66.9% are diagnosed by 24 hours 1
• 94.5% are captured by 48 hours, making the traditional 72-hour protocol unnecessarily prolonged in most cases 1
• Elevated immunoreactive proinsulin at the beginning of the fast is present in 90% of insulinoma patients, providing early diagnostic support 1

When to Use the Mixed-Meal Tolerance Test (MMTT)

If the 72-hour fast is negative but the patient has predominantly postprandial neuroglycopenic symptoms occurring 1-4 hours after eating, proceed directly to a mixed-meal tolerance test. 3, 5

MMTT Protocol and Interpretation

• Administer a mixed meal containing carbohydrates, fats, and proteins after an overnight fast 2
• Collect blood samples before meal ingestion and at 30-minute intervals for up to 180 minutes (some protocols extend to 2 hours) 2
• Monitor for development of hypoglycemia (glucose typically <55 mg/dL) between 60-180 minutes post-ingestion 2
• Measure concurrent insulin and C-peptide levels during any hypoglycemic episode 2

Critical Diagnostic Distinction

The MMTT is positive when hypoglycemia develops in the postprandial window (1-3 hours after eating) with inappropriately elevated insulin and C-peptide levels. 2 This pattern suggests:

• Non-insulinoma pancreatogenous hypoglycemia syndrome (NIPHS): characterized by islet hypertrophy/nesidioblastosis rather than discrete tumor, exclusively postprandial symptoms, and negative 72-hour fasts 5
• Post-bariatric surgery hypoglycemia (late dumping syndrome): occurs 1-3 hours after meals in patients with prior gastric/bariatric surgery 2

Important Clinical Caveats

• The oral glucose tolerance test (OGTT) should NOT be used for diagnosing postprandial hypoglycemia—the Endocrine Society explicitly recommends against it due to low diagnostic accuracy and frequent false positives in asymptomatic individuals 2
• The mixed-meal test is more physiologic than OGTT but lacks firmly established normative values in healthy controls, so clinical correlation with symptoms is essential 2
• Always exclude exogenous insulin or sulfonylurea use: low C-peptide during hypoglycemia indicates exogenous insulin injection, while positive sulfonylurea screen confirms oral agent ingestion 2
• In patients with prior upper GI surgery presenting with postprandial symptoms, consider alternative diagnoses including marginal ulcer, stenosis, internal herniation, or gallstone disease before attributing symptoms to hypoglycemia 2

Algorithmic Summary

1. Patient presents with suspected hyperinsulinemic hypoglycemia → First determine symptom timing
2. Fasting or meal-independent symptoms → 72-hour (practically 48-hour) supervised fast with insulin, C-peptide, proinsulin, glucose, beta-hydroxybutyrate, and sulfonylurea screen 1, 3
3. Exclusively postprandial symptoms (1-4 hours after eating) → Mixed-meal tolerance test with serial glucose, insulin, and C-peptide measurements 3, 5
4. Positive 48-hour fast → Proceed to imaging (multiphasic CT/MRI, endoscopic ultrasound) and consider selective arterial calcium stimulation for insulinoma localization 4
5. Negative 48-hour fast but positive MMTT → Consider NIPHS; selective arterial calcium stimulation may show diffuse pancreatic beta-cell hyperfunction rather than focal lesion 5