72 hour fast information sheet

You are booked to come in for a : PROLONGED SUPERVISED FAST

What is a prolonged supervised fast?

A prolonged supervised fast looks to see what happens to the sugar levels in your blood stream when you fast. This can give clues about symptoms that you may have experienced that could be caused by a low blood sugar. The test also provides information about your body’s production of a hormone called insulin. A hormone is a chemical which is made in one part of the body but passes into the bloodstream and has effects on other parts of the body. Insulin is a hormone produced by the pancreas gland which acts on several body tissues to affect uptake of sugar. When things are working normally, if your blood sugar levels are low your body does not release much insulin. The prolonged fast helps to test if your body is producing more insulin than it should.

How should I prepare for the test?

You should eat normally in the days leading up to the test. You should not fast before the test. You should take your medications as usual the day before the test unless your doctor has advised you not to.

What will happen during the test?

You should come to the ward on the date of the test, by 9am. You will have a bed to lie down in throughout the test. A cannula (a small needle with a tube attached for taking blood samples) will be placed in a vein in your arm. You are not allowed to eat anything during the test, although you are permitted to drink water or other calorie-free drinks provided by the nursing staff. Blood samples will be taken at regular intervals from the cannula during the test. The fast lasts for up to 72 hours. If you have had no symptoms during that time you may be asked to do some exercise (such as a brisk walk around the hospital) before a final set of blood tests. Once the test is completed, the cannula will be removed. Once you have had something to eat and drink and are feeling fine you will be able to go home.

What happens after the test?

When the results of the blood tests are available they will be reviewed by the endocrinology team. You will be notified by letter and your GP will also be informed of the results. If there are any changes to be made to your treatment or further tests that need to be organized, these will be outlined in the letter or someone from the department may contact you by telephone.

What if I have any questions?

You will see a doctor on the day of your test who should be able to answer your queries. If you have any questions before or after your test please leave a message and contact number with the endocrine department at the Hospital where your test is booked. Someone will call you back to discuss things.


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