What is the role of the 72-hour fasting plasma glucose test in the diagnosis of hypoglycemia (low blood sugar)?

Updated: Sep 12, 2019
  • Author: Osama Hamdy, MD, PhD; Chief Editor: Romesh Khardori, MD, PhD, FACP  more...
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Answer

A supervised fast is the most reliable diagnostic test for the evaluation of fasting hypoglycemia. Continue the fast for as long as 72 hours or until symptoms develop in the presence of hypoglycemia (blood sugar < 45 mg/dL (2.5 mmol/L) for women; < 55 mg/dL (3.05 mmol/L) for men). Obtain simultaneous insulin levels every 6 hours, when glucose is low and when symptoms develop. Also measure the beta-hydroxybutyrate serum level. Glucose and/or glucagon must be administered after blood sample withdrawal to abort hypoglycemic symptoms. The diagnosis of insulinoma is likely if the patient, at the conclusion of the fast, has neuroglycopenic symptoms, a fall in plasma glucose to less than 45 mg/dL (< 2.5 mmol/L), inappropriately elevated beta-cell polypeptides (insulin, proinsulin, and C-peptide levels), and a beta-hydroxybutyrate level of less than 2.7 mmol/L.

For overnight fasting plasma glucose levels, symptoms of hypoglycemia may develop when the blood sugar is below 60 mg/dL (3.33 mmol/L).


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