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