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

Who is a candidate for the test?

The test is done when a doctor thinks a person has diabetes mellitus, which results in a high blood glucose level. 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 certain times after the person drinks a solution of glucose in water.

A blood sample is first taken before the person drinks a sweet drink. Then, samples are taken again at certain times after drinking it. Common times that are used include:

To measure glucose in the blood, a sample of blood is needed. This is most often 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 rubber tube called a tourniquet is wrapped around the upper arm. This enlarges the veins in the lower arm and restricts 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 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 After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.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, are collected at the same time intervals as the blood samples are taken.