What labs should be ordered for a person with hypoglycemia (low blood sugar) who is eating every 2 hours and drinking 3/4 gallon of water a day?

Laboratory Tests for Hypoglycemia with Frequent Eating and Increased Water Intake

For a person with hypoglycemia who is eating every 2 hours and drinking 3/4 gallon of water daily, comprehensive laboratory testing should include fasting plasma glucose, insulin levels, C-peptide, and a 72-hour fast to determine the underlying cause of hypoglycemia.

Initial Laboratory Evaluation

• Blood glucose measurement to confirm hypoglycemia (blood glucose ≤70 mg/dL or 3.9 mmol/L) and establish Whipple's triad (symptoms, low blood glucose, and resolution of symptoms with glucose administration) 1
• Complete metabolic panel including:
  • Electrolytes (sodium, potassium, chloride, bicarbonate) 2
  • Kidney function tests (BUN, creatinine, eGFR) 2
  • Liver function tests (AST, ALT, bilirubin) 3
• Hemoglobin A1C to assess long-term glycemic control 2

Hormonal Assessment

• Insulin level during hypoglycemic episode to identify hyperinsulinemia 1
• C-peptide level to differentiate endogenous from exogenous insulin 1
• Cortisol level to evaluate adrenal function and stress response 4
• Growth hormone to assess for growth hormone deficiency 5
• Insulin antibodies to check for autoimmune hypoglycemia syndrome 1

Specialized Testing

• 72-hour supervised fast if fasting hypoglycemia is suspected, with serial measurements of glucose, insulin, C-peptide, and beta-hydroxybutyrate 1
• Mixed meal tolerance test if postprandial (reactive) hypoglycemia is suspected 1
• Urine testing for ketones to evaluate for starvation ketosis, especially with the patient's frequent eating pattern 2

Additional Considerations

• Urine glucose to assess for renal threshold for glucose 2
• Serum uric acid levels, especially if diazoxide treatment is being considered 3
• Complete blood count to monitor for potential medication side effects 3

Interpretation Based on Clinical Context

The pattern of frequent eating (every 2 hours) suggests the patient may be experiencing recurrent hypoglycemic episodes and is attempting to prevent them through constant carbohydrate intake 2. The high water intake (3/4 gallon daily) could be related to:

• Attempt to maintain hydration during illness 2
• Medication side effect causing thirst 3
• Possible diabetes insipidus or other conditions causing polydipsia 4

Common Pitfalls to Avoid

• Don't rely solely on random glucose measurements - hypoglycemia should be documented during symptomatic episodes 1
• Don't overlook factitious hypoglycemia - surreptitious insulin or sulfonylurea use should be considered 6
• Don't miss non-islet cell tumors - these rare tumors can cause hypoglycemia through IGF-II production 1
• Don't forget medication review - many medications can cause hypoglycemia 5

Follow-up Testing

After initial evaluation, additional testing may be needed based on results:

• Imaging studies (CT, MRI) if insulinoma or other tumors are suspected 1
• Continuous glucose monitoring to detect patterns of hypoglycemia, especially nocturnal episodes 2
• Repeat testing after temporary avoidance of hypoglycemia (2-3 weeks) if hypoglycemia unawareness is present 2

Remember that the frequent eating pattern may be masking more severe hypoglycemia, and laboratory testing should ideally be performed during a spontaneous hypoglycemic episode or under controlled conditions 1.