Oral glucose tolerance test (adults)

What is an oral glucose tolerance test?

This is a procedure where blood samples are collected before and after a glucose drink for measurement of blood glucose (sugar) levels. This procedure is usually performed to help establish a diagnosis of diabetes mellitus (sugar diabetes).

Are there any risks?

You will be given a very sweet drink which may make you feel nauseous (sick).

What should I do before I come for the test?

Please follow these instructions carefully. Failure to do so will make it difficult for your doctor to interpret your results.

• You must have fasted for at least eight hours, and not more than 14 hours. This means that from the night before your test, you must not eat anything and must drink only water. Substances in tea, coffee, or cola can affect the results, so even black coffee and sugar-free (diet) drinks must be avoided.

• If you have medicine to take in the morning of your test you should still take this, unless told otherwise by your doctor. If your morning medicine needs to be taken with food, please discuss this with your doctor.

• You should not chew gum, smoke, or wear a nicotine patch on the morning of your test.

• You may want to bring a book, magazine, or electronic device to read.

What will happen when I have the test?

A blood sample will be taken from a vein in your arm. Then your fasting blood sugar level will be checked using a sample taken by pricking your finger. If the sugar level in the finger-prick sample is found to be high, the test will be stopped and you will be advised to return to your GP.

If the sugar level in the finger-prick sample is not high the test will continue. You will be given a very sweet drink which contains an exact amount of sugar (glucose). This must be drunk within five minutes and then a further drink of plain water. In the unlikely event that you are sick after having this drink you must tell staff immediately as the test will not be continued.

You will be expected to sit quietly for two hours close to where the test is carried out. You must not leave to go for a walk or to go shopping. You may drink water during the test but you must not eat, smoke, or chew gum.

After two hours a second blood sample will be taken from a vein in your arm. The two blood samples will be sent to the laboratory to measure the amount of glucose in them.

This is the end of the test and you will be able to eat and drink normally again. You will be free to go home.

How long will I be in hospital?

Once the test has started it will take about two hours, after which you can go home.

How will I find out my results?

The results of your test will be sent to the doctor or midwife who requested the test.

Where can I get more information?

If you have any further queries about why this test is being done, please speak to your doctor or the healthcare professional who requested the test.