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glucose tolerance test

Alternative Names
oral glucose tolerance test, OGTT, GTT

This test is primarily used to measure how well the body can use, or metabolise, glucose. Glucose is the main form of sugar in the body.

Who is a candidate for the test?
The test is performed on a person who is suspected of having diabetes mellitus, or high blood sugar. It is also advised in all pregnant women to screen for diabetes of pregnancy.

How is the test performed?
In the glucose tolerance test, samples of a person's blood are measured at specified intervals after the person drinks a solution of glucose in water.

A blood sample is first taken before the person drinks the glucose solution. Samples are taken again at certain intervals after swallowing the solution. Common times used include 30 minutes, 1 hour, 2 hours, and 3 hours after swallowing the glucose solution.

To measure glucose in the blood, a sample of blood is needed. This is usually drawn from a vein in the forearm or hand. To do this, the skin over the vein is first cleaned with an antiseptic. Next, a strong rubber tube, or tourniquet, is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them. A very thin needle is gently inserted into a vein and the tourniquet is removed. Blood flows from the vein through the needle and is collected in a syringe or vial. It is then sent to the laboratory for glucose testing. After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.

In some cases, urine samples may also be used to look for extra glucose. Samples of midstream urine, that is, not at the beginning and not at the end, is collected at the same time intervals as the blood samples are taken.

What is involved in preparation for the test?
People are often advised not to eat for at least 8 hours before the test. Certain medications, such as diabetes medications, may be stopped before the test. A person should check with the doctor for specific instructions.

What do the test results mean?
The normal value of blood glucose after not eating for 8 hours is less than 7.0 mmols/L (millimoles per litre). Various blood glucose levels are considered normal at each time interval after swallowing the glucose. This test is normally done to test for diabetes; however, other conditions can cause abnormal results.

If levels are lower than normal, the person may have:
  • hypoglycaemia, or low blood sugar
  • bowel problems that interfere with absorbing the glucose into the body
  • certain hormone imbalances, such as a low thyroid hormone level
  • a tumour that secretes too much insulin, which is rare
  • certain drugs in their system that interfere with the test, such as caffeine
If levels are higher than normal, the person may have:
  • diabetes mellitus, which is blood sugar that is too high. Some cases of diabetes are permanent, whereas others may be related to a pregnancy and go away after delivery of the child.
  • hormone imbalances. These include a high thyroid hormone level or a high level of cortisol, which is a hormone that is important in metabolism.
  • damage to the pancreas, which is the organ that secretes insulin. Insulin is a hormone that helps keep the blood sugar from getting too high. When damaged, the pancreas may not be able to secrete enough insulin.
  • certain tumours, such as ones that secrete adrenaline, a stress hormone.
  • certain drugs in their system that interfere with the test, such as corticosteroids, or steroids.
Author: David T. Moran, MD
Reviewer: HealthAnswers Australia Medical Review Panel
Editor: Dr David Taylor, Chief Medical Officer HealthAnswers Australia
Last Updated: 1/10/2001
Potential conflict of interest information for reviewers available on request

This website and article is not a substitute for independent professional advice. Nothing contained in this website is intended to be used as medical advice and it is not intended to be used to diagnose, treat, cure or prevent any disease, nor should it be used for therapeutic purposes or as a substitute for your own health professional's advice.  All Health and any associated parties do not accept any liability for any injury, loss or damage incurred by use of or reliance on the information.


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