Prolonged Supervised Fast

INDICATION

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

PREPARATION

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.

METHOD

• 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.

INTERPRETATION

• 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.

SENSITIVITY AND SPECIFICITY

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.

REFERENCE

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