Prolonged glucose tolerance test

Investigation Protocol


Diagnosis of reactive hypoglycaemia.


Glucose solution is best prepared in advance as glucose dissolves better in warm water but is more palatable when cold. On the day prior to the test, dissolve 75 grams of anhydrous glucose (82.5 grams of glucose monohydrate, obtainable pre-weighed from pharmacy) in warm water and store in a fridge overnight.

The patient must be fasted from midnight (sips of water only).


Check that the patient has fasted from midnight.

Insert an indwelling cannula gauge 20 with a three-way tap. Using the vacutainer connector system or a syringe, fill a fluoride oxalate bottle with 2mls of blood, label the tube with time=0, and test a small sample using a glucose meter.

Give the patient the glucose solution to drink (within 10 minutes).

Take a further blood sample, test it using a glucose meter, and place the rest of the sample into a fluoride oxalate bottle at 30, 60, 90, 120, 150, 180, 240 minutes (total test time four hours).

If any glucose meter readings are below 3.0mmol/l during the test, take two further samples at 270 and 300 minutes.

Record any symptoms the patient has with the times in the notes.

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Patient information sheet

Prolonged glucose tolerance test

Your doctor has referred you to have a prolonged glucose tolerance test, which is normally performed over a four hour period. This will establish what happens to your blood glucose (sugar) levels after you have consumed a high volume of sugar. 

During the test blood samples will be taken at half hourly intervals. When you arrive in the unit we will insert a small tube (a cannula) into a vein in your arm, which will allow us to take the blood samples more easily. After the first samples of blood have been taken, you will be given chilled glucose solution to drink over ten minutes. You will then be able to relax in between samples of blood being taken. You will be resting on a reclining chair.

It is important that you do not have anything to eat or drink from midnight, except sips of water. You will be offered a drink and something to eat at the end of the test.

We will be monitoring you for any symptoms during the test. If you become symptomatic and your blood glucose level starts to drop we may need to extend the test to five hours.

Smoking should be avoided on the day of the test. You should take your regular medication as normal unless we advise you otherwise.

You should expect your stay at the hospital to last at least four and half to five and a half hours, depending on your results. 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). You may bring someone to stay with you during the test but there is not enough space for more than one person.

Children are discouraged, as this can be disruptive to other patients. However, if this poses a particular problem for you please discuss this with the senior nurse in the unit. 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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