What laboratory workup is recommended for evaluating unexplained hypoglycemia in a non‑diabetic patient?

Laboratory Workup for Unexplained Hypoglycemia in Non-Diabetic Patients

The diagnostic evaluation of spontaneous hypoglycemia in non-diabetic adults must begin by documenting Whipple's triad during a symptomatic episode, followed by a supervised 72-hour fast with critical laboratory measurements to differentiate between hyperinsulinemic and non-hyperinsulinemic causes. 1, 2, 3

Step 1: Confirm True Hypoglycemia with Whipple's Triad

Before pursuing any laboratory workup, you must document all three components of Whipple's triad 2, 3, 4:

• Symptoms or signs consistent with hypoglycemia (neurogenic symptoms: sweating, trembling, palpitations, anxiety, hunger; neuroglycopenic symptoms: confusion, difficulty concentrating, slurred speech, visual changes, behavioral changes, seizures) 5
• Low plasma glucose concentration (documented laboratory glucose <55 mg/dL, not just fingerstick readings) 3, 4
• Resolution of symptoms when glucose is raised 1, 2, 3

Critical pitfall: Many patients are labeled "hypoglycemic" based on symptoms alone or fingerstick readings without laboratory confirmation. This leads to unnecessary workups in healthy individuals. 4

Step 2: Obtain Critical Labs During Symptomatic Hypoglycemia

When the patient is symptomatic and glucose is documented <55 mg/dL, immediately draw blood for the following "critical sample" 1, 2, 3:

• Plasma glucose (laboratory measurement, not fingerstick) 3, 4
• Insulin 1, 2, 3
• C-peptide 1, 2, 3
• Proinsulin 1, 2, 3
• Beta-hydroxybutyrate 1, 2, 3
• Plasma and urine sulfonylurea/meglitinide screen 1, 3
• Insulin antibodies (if insulin autoimmune syndrome suspected) 3, 6

These labs will classify hypoglycemia into three categories that direct further investigation 4:

• Non-ketotic hyperinsulinemia (elevated insulin, C-peptide, suppressed beta-hydroxybutyrate)
• Non-ketotic hypoinsulinemia (low insulin, C-peptide, suppressed beta-hydroxybutyrate)
• Ketotic hypoinsulinemia (low insulin, C-peptide, elevated beta-hydroxybutyrate)

Step 3: Supervised Provocative Testing

If spontaneous hypoglycemia cannot be captured, perform supervised testing based on symptom timing 1, 2, 3:

For Fasting or Random Symptoms: 72-Hour Supervised Fast

• Gold standard test for evaluating suspected endogenous hyperinsulinism 1, 2, 3
• Patient fasts under medical supervision with serial glucose monitoring every 4-6 hours (more frequently if glucose <60 mg/dL) 3
• When glucose falls to <55 mg/dL with symptoms, obtain the critical sample listed above 1, 3
• Test is terminated when glucose <55 mg/dL with symptoms, or after 72 hours 3

For Postprandial Symptoms: Mixed Meal Test

• Preferred for patients with predominantly postprandial symptoms (occurring 1-5 hours after meals) 1, 2
• Administer standardized mixed meal and monitor glucose every 30 minutes for 5 hours 1
• Obtain critical sample when glucose <55 mg/dL with symptoms 1

Step 4: Interpret Results and Direct Further Workup

If Hyperinsulinemic (Elevated Insulin/C-peptide, Suppressed Beta-hydroxybutyrate):

With elevated C-peptide (endogenous insulin) 1, 3, 6:

• Negative sulfonylurea screen: Proceed to imaging for insulinoma (CT/MRI pancreas, endoscopic ultrasound, selective arterial calcium stimulation) 1, 3
• Positive sulfonylurea screen: Factitious hypoglycemia from oral hypoglycemic agents 1, 3
• Consider post-bariatric hypoglycemia if history of gastric bypass surgery 2, 6
• Consider non-insulinoma pancreatogenous hypoglycemia syndrome (NIPHS) if imaging negative 1, 2

With suppressed C-peptide but elevated insulin 3, 6:

• Exogenous insulin administration (factitious) 1, 3
• Check insulin antibodies for insulin autoimmune syndrome (Hirata disease) 3, 6

If Non-Hyperinsulinemic (Low Insulin/C-peptide):

With suppressed ketones 4:

• Non-islet cell tumor hypoglycemia (measure IGF-1, IGF-2, Big-IGF-2) 1, 6
• Evaluate for large mesenchymal or epithelial tumors 6

With elevated ketones 4:

• Cortisol deficiency: Check 8 AM cortisol, ACTH stimulation test 1, 3, 6
• Growth hormone deficiency: Consider if hypopituitarism suspected 3, 6
• Inborn errors of metabolism (rare in adults): Consider if hepatomegaly, cardiomyopathy, or exercise-induced symptoms 6

Step 5: Exclude Common Causes Before Extensive Testing

Before pursuing provocative testing, systematically exclude 2, 3:

• Medications: Insulin, sulfonylureas, meglitinides, fluoroquinolones, pentamidine, quinine 1, 3
• Alcohol use: Especially with poor nutritional intake 3, 6
• Critical illness: Sepsis, liver failure, renal failure, cardiac failure 2, 3
• Organ failure: Hepatic dysfunction (check liver function tests), renal dysfunction (check creatinine) 2, 3

Key Pitfalls to Avoid

• Do not pursue workup without documented Whipple's triad - this is the most common error leading to misdiagnosis of healthy individuals 3, 4
• Do not rely on fingerstick glucose alone - laboratory plasma glucose confirmation is mandatory 4
• Do not obtain labs when patient is asymptomatic and normoglycemic - results will be uninterpretable 2, 4
• Do not forget to screen for sulfonylureas/meglitinides - factitious hypoglycemia is more common than insulinoma 1, 3
• Do not assume "reactive hypoglycemia" without documentation - this diagnosis is overused and rarely confirmed with proper testing 2, 4