72-Hour Fast

The 72-hour fast is the definitive test for confirming or excluding endogenous hyperinsulinaemic hypoglycaemia (insulinoma, nesidioblastosis, post-bariatric hypoglycaemia) in adults with Whipple triad: symptoms of hypoglycaemia, low plasma glucose at the time, and relief of symptoms when glucose is raised. During fasting, normal people suppress insulin as glucose falls. In insulinoma, insulin secretion continues despite hypoglycaemia.

The test also separates other causes, including factitious hypoglycaemia with insulin or sulphonylureas, insulin autoimmune syndrome and non-islet tumour hypoglycaemia (IGF-2).

Insulinoma has an incidence of 1 to 4 cases in every million people each year, mostly in adults aged 40 to 60 years, and 90% are benign and solitary (under 2 cm). The 72-hour fast is positive in over 99% of insulinoma by 72 hours, and in about 65% by 24 hours, 85% by 48 hours. Severe prolonged hypoglycaemia causes neuroglycopenic injury, and cure by surgical resection is possible in most cases.

Indications. Documented spontaneous hypoglycaemia (plasma glucose under 3.0 mmol/L) with symptoms and no other explanation, in an adult. Recurrent hypoglycaemia in a patient not on glucose lowering drugs. Suspected insulinoma or factitious hypoglycaemia. The test is not done in patients with Whipple triad already documented with a critical sample showing hyperinsulinism.

Contraindications and cautions. Not done in outpatients, since it needs supervision and frequent samples. Caution in patients with seizures, cardiac disease or adrenal insufficiency, which should be excluded first. Pregnancy and children need specialist regimens. Medications that cause hypoglycaemia are reviewed beforehand.

Preparation. Admission to hospital. Baseline cortisol, TSH and routine biochemistry. Review of drugs. Discontinuation of non-essential drugs. Diet is normal until the fast begins (after a late evening meal, or at a time that allows observation). The patient may drink water and non-caloric, caffeine free fluids, and is kept active during the day.

Procedure.

• Start time is recorded (last food). Cannula placed. Samples for plasma glucose, insulin, C-peptide, proinsulin and beta hydroxybutyrate are taken at the start and every 6 hours.
• When glucose is under 3.3 mmol/L (60 mg/dL), sampling is every 1 to 2 hours. Bedside glucose meters are used for screening, but laboratory plasma glucose is the reference.
• Fast ends when plasma glucose is under 2.5 mmol/L (45 mg/dL) with symptoms or signs of hypoglycaemia, or when the 72 hours are complete (some centres stop at 48 hours if negative and exercise is added), or when there is unequivocal Whipple triad.
• At the end: take samples for glucose, insulin, C-peptide, proinsulin, beta hydroxybutyrate, cortisol and a sulphonylurea screen.
• Glucagon 1 mg intravenously is given at the end, with plasma glucose measured at 10, 20 and 30 minutes. A rise of 1.4 mmol/L (25 mg/dL) or more shows glycogen stores preserved by insulin action.
• Then the patient is fed, and hypoglycaemia is treated with oral or intravenous glucose.

Diagnostic criteria for endogenous hyperinsulinism (at plasma glucose under 3.0 mmol/L, usually under 2.5 mmol/L).

• Insulin 3 microU/mL (18 pmol/L) or more (6 microU/mL by some assays).
• C-peptide 0.6 ng/mL (0.2 nmol/L) or more.
• Proinsulin 5 pmol/L or more.
• Beta hydroxybutyrate 2.7 mmol/L or less (suppressed ketones).
• Glucose rise 1.4 mmol/L or more after glucagon.
• Sulphonylurea and meglitinide screen negative.

Patterns.

• Insulinoma: all of the criteria listed, usually with symptoms by 48 hours, and imaging (endoscopic ultrasound, CT, MRI, 68Ga-DOTATATE or exendin-4 PET) localises the tumour.
• Exogenous insulin (factitious): high insulin, low C-peptide and low proinsulin, with suppressed ketones. Insulin analogues may not be detected by all immunoassays.
• Sulphonylurea or meglitinide: high insulin, C-peptide and proinsulin with positive drug screen.
• Insulin autoimmune syndrome: very high insulin (over 1000 microU/mL), positive insulin antibodies, usually with other autoimmune disease or drug triggers (methimazole, alpha lipoic acid).
• Non-islet cell tumour hypoglycaemia: low insulin, C-peptide and proinsulin, suppressed ketones, raised IGF-2 to IGF-1 ratio (over 10), and a mass (mesenchymal tumours).
• Appropriate fasting response: ketones raised (beta hydroxybutyrate over 2.7 mmol/L), glucose may fall to 2.5 to 3.0 mmol/L without symptoms in healthy women and children after prolonged fasting, with insulin suppressed (under 3 microU/mL).
• Adrenal insufficiency, growth hormone deficiency, hepatic failure and sepsis: low insulin with appropriate ketone response.

Critical values and findings needing immediate action.

• Plasma glucose under 2.5 mmol/L with neuroglycopenic symptoms: end the fast immediately, take final samples and give intravenous 50% dextrose or glucagon.
• Seizure, coma or arrhythmia during the fast.
• Confirmed insulinoma: urgent endocrine and surgical referral, with diazoxide or octreotide as bridging.
• Suspected factitious hypoglycaemia: handled with sensitivity and involves psychiatric and safeguarding evaluation.

Interferences and pitfalls.

• Glucose must be measured in fluoride oxalate tube (or immediately), because glycolysis in whole blood lowers glucose by 5 to 7% every hour.
• Insulin assays differ, and some do not detect analogues. Haemolysed samples degrade insulin.
• Inadequate fasting (food or sugar containing drinks) invalidates the test.
• Sulphonylureas have long half lives, and the drug screen is taken at the time of hypoglycaemia.
• Post-bariatric hypoglycaemia occurs after meals and is diagnosed by a mixed meal test, not a fast.
• A normal 72-hour fast excludes insulinoma and directs evaluation to postprandial causes.
• Exercise during the fast raises sensitivity but needs supervision.
• Hepatic and renal disease, hypopituitarism and adrenal insufficiency must be excluded before the test.
• Imaging is not done unless biochemistry is positive.

Special groups.

• Children: hypoglycaemia workup uses a critical sample at the time of hypoglycaemia, not a 72-hour fast.
• Older adults: factitious hypoglycaemia and drug induced causes are frequent.
• Pregnancy: fast is avoided.
• Regions: factitious cases occur in healthcare workers and relatives of diabetic patients.

Follow up tests. Pancreatic imaging (endoscopic ultrasound, contrast CT or MRI, DOTATATE or exendin PET), multiple endocrine neoplasia type 1 screen (calcium, PTH, prolactin, pituitary MRI, MEN1 gene), insulin antibodies, sulphonylurea screen, IGF-2, cortisol, growth hormone, chromogranin A, and intraoperative ultrasound for localisation.