Oral glucose tolerance test

Investigation Protocol


The diagnosis of diabetes, impaired glucose tolerance, insulin resistance and reactive hypoglycaemia. OGTT for the diagnosis of acromegaly is listed separately as growth hormone suppression test.


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.

Perform venepuncture taking 2mls of blood; test a small sample using a near patient glucose testing meter. Other baseline bloods should be taken if requested by referring clinician.

Place the remainder of the blood sample into a fluoride oxalate bottle and label with patient identification, date and ‘0’ minutes in time on sample.

If the result on the glucose meter is greater than or equal to 11mmol/l, send the blood sample urgently to lab. If it is confirmed by biochemistry to be above 11mmol/l, there is no need to continue test, and the patient can go home.

If the result is less than 11mmol/l on meter, give the patient the glucose solution to drink (within 10 minutes). Collect a further blood sample in a fluoride oxalate bottle for glucose measurement two hours after the glucose solution has been given. Blood samples must be labelled with patient details, date and time of sample (0 and 120 minutes).

Send samples all together with request form to laboratory.

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Patient information sheet

Oral glucose tolerance test

Your doctor has referred you to have an oral glucose tolerance test which is performed over a two hour period. This is to establish whether or not you are developing diabetes, or whether your body has difficulty in using glucose (sugar) properly. 

We will take a sample of blood when you arrive in the unit. This will be tested for glucose concentration level, if this is high it will be unnecessary for the test to continue. Provided that the glucose level is not too high you will be given a drink of chilled glucose solution to be drunk over ten minutes. Following this a further sample of blood will be taken after two hours. You will be resting in a reclining chair in the unit throughout the test.

For three days prior to this test you should consume a normal diet. If you have been put on a weight reducing or low carbohydrate diet this must be stopped for three days. On the day of the test, 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. You are also advised not to smoke on the day of the test. If you are taking any medicines or pills please check with our department to see if you should take them on the day of the test.

You should expect to stay at the hospital for about two and half hours. 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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