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Prolonged Supervised Fast


Used to demonstrate fasting hypoglycaemia and diagnose insulinoma if not shown spontaneously or after an overnight fast.


Admit to perform test under close supervision with glucose (p.o./i.v.) available.
Leave a copy of this protocol sheet in the nurses' notes and a copy above the patient's bed.


  • Cannulate patient and commence 72 hr fast.
  • Water/non-caloric beverages allowed. Patient should be active during waking hours.
  • Blood glucoses should be done at regular (4-6 hr) intervals and whenever the patient has symptoms suggestive of hypoglycaemia. Decrease to 2 hr intervals if the patient consistently has glucoses <3.0 mmol/l.
  • If blood glucoses are =2.2 mmol/l or symptoms are convincing:
    • Bleep endocrine SHO urgently.
    • Take blood for glucose, insulin and C-peptide in a plain clotted tube (7 ml) and a fluoride oxalate tube.
    • Take blood and spot urine for sulphonylurea screen in a plain clotted tube (7 ml) and a Sterilin universal container.
    • Take to chemistry labs to be separated and frozen within 30 mins. Ring biochemistry up for an urgent glucose.
    • Do not reverse hypoglycaemia until the lab confirms hypoglycaemia, or unless the patient becomes unconscious or fits.
    • If no symptoms during the fast, finish with 15-30 mins exercise, e.g. a brisk walk around the hospital.
    • Take final samples for glucose, insulin and C-peptide, sulphonylurea screen.


  • Normals do not become hypoglycaemic, although young women can run glucoses in the region of 2.2-3.0 without symptoms.
  • True hypoglycaemia must be demonstrated (glucose =2-2.2 mmol/l), before we are able to either interpret insulin results or consider insulinoma.
  • If hypoglycaemia with raised insulin but low C peptide, consider self administration of insulin.
  • If hypoglycaemia with raised insulin, and raised C-peptide, make sure sulphonylurea screen is negative!
  • With hypoglycaemia, insulin and endogenous insulin production (estimated by C-peptide) should be low.
    • Insulin >6 mU/l (>50 pmol/l); C peptide >300 pmol/l = insulinoma (check ratio of c-peptide to insulin high enough).
    • Insulin >3-6 mU/l (25-50 pmol/l); C peptide 100-300 pmol/l = possible insulinoma but needs further tests
    • Insulin <3 mU/l (<25 pmol/l); C peptide <75 pmol/l = normal response
  • Ketones should be suppressed with insulinoma even though patient is fasting because of the excess insulin.


By 24 hrs, 66% insulinomas develop hypoglycaemia and by 48 hrs, >95% insulinomas can be diagnosed. After 72 hrs fast plus exercise, if no hypoglycaemia, insulinoma is very unlikely.


Friesen, S.R. Surg. Clin. N. Amer. 67(2). 379 (1987).