Supervised 72 hour fast

Investigation Protocol


To confirm and determine the cause of suspected spontaneous hypoglycaemia <2.2mmol/l.

Precautions and preparation

Pregnancy, terminal disease for example malignancy or liver disease, and renal failure are contra-indications to this test. All patients must be warned that they will be fasting, and unable to leave the ward unaccompanied during the test before starting.


The patient should eat and drink normally prior to admission. The fast is usually started at 6pm although some units will perform a day fast on patients with very high suspicion of insulinoma, in which case it starts at 9am.

The patient is allowed nothing to eat or drink except water from the start of the fast. The patient can take their medication as usual, with water only. The patient is not allowed to leave the ward unaccompanied during the fast, and needs to be visible from the nurses’ station. This is for patient safety, but also to minimise the risk of illicit insulin or sulphonylurea use.

At the start of the fast, take blood into a fluoride oxalate tube for glucose, and a lithium heparin tube. The lithium heparin tube will need to be centrifuged immediately and the plasma frozen for future analysis of insulin and c peptide. 

Commence two hourly near patient testing of capillary blood glucose. If the glucose is below 3.3mmol/l, take further samples in both fluoride oxalate and lithium heparin bottles, and arrange immediate analysis of venous glucose. If this is confirmed below 2.2mmol/l, ensure that the lithium heparin sample has been taken, and terminate the fast. Emergency treatment for hypoglycaemia should be given if clinically required - with fruit juice, lucozade, food, or if necessary with slow intravenous glucose (0.4mls 50% dextrose/kg = 28mls for a 70kg adult).

If the laboratory venous glucose is above 2.2mmol/l, continue the fast. Continue two hourly glucose monitoring, and repeat formal blood tests in a fluoride oxalate and lithium heparin tube every four hours, or immediately if capillary blood glucose is <3.3mmol/l, throughout the supervised fast. If the glucose levels are confirmed as normal, not all of htese samples will be assayed, though they should still be collected. This will be determined by the requesting consultant at the end of the test.

Once hypoglycaemia is confirmed, also take a specimen of urine in a plain pot for a sulphonylurea screen to be performed.

If the patient has no confirmed hypoglycaemia during 72 hours, they can be discharged.

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Patient information sheet

Supervised 72 hour fast

Your doctor has referred you to be admitted for a supervised fast to establish whether your blood glucose (sugar) levels drop when you don’t eat. The test starts at 6pm and lasts for up to 72 hours. You will be allowed to drink water and to take your usual medication during the test, but you will not be allowed to eat or drink anything else during this time. You will also be expected to remain in the department while the test takes place, so that a member of staff can supervise you at all times.

We will monitor your sugar levels by measuring a finger prick blood sample in a special meter. If the sugar levels drop significantly we will take samples of blood from a vein in your arm to send to the laboratory. If we need to take regular samples of blood we may insert a small tube (a cannula) into a vein in your arm to enable us to obtain samples more easily. You will mostly be resting on a reclining chair but may be asked during the day to take some exercise up and down the stairs nearby.

Please eat and drink normally before the start of the test. You will be offered a drink and something to eat at the end of the test.

You should expect your stay at the hospital to last 72 hours, though you may be allowed home earlier, if you do develop a low blood sugar.

You may wish to bring a book or magazine to read. We do have a radio, or you can access Patientline; alternatively you may like to bring your own personal radio/cd player (please bring your own headphones). Visitors will be allowed during the test, but you will not be allowed to leave the ward unaccompanied throughout the test. At the end of the test, you will be given a full meal and allowed home. You should be able to undertake normal activities after the test is finished and it should not interfere with your ability to drive home. 

If you have any queries about the test, please contact: 

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