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Oral Glucose Tolerance Test - Glucose Tolerance Test
Overview & Description
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 After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.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.
Preparation & Expectations
What is involved in preparation for the test?
People are often advised not to eat for at least 8 hours before the test. This is called fasting. Certain medicines, such as those for diabetes, may be stopped before the test. A person should check with the doctor for specific instructions.
Results and Values
What do the test results mean?
Normal fasting blood glucose levels are less than 110 milligrams/deciliter, or mg/dL. If a person does not have diabetes, the glucose levels will rise and then fall quickly after drinking the sweet liquid. When a person has diabetes, glucose levels will rise higher and fail to come down as fast as those in a person without diabetes. Healthy blood glucose levels after drinking the glucose are less than 140 mg/dL. If the blood glucose level is 140 to 199 mg/dL 2 hours after drinking the liquid, a diagnosis of prediabetes can be made. If the blood glucose level is higher than 200 mg/dL, then another test is done on a different day to confirm whether the person has diabetes.
If levels are lower than normal, the person may have:
If levels are higher than normal, the person may have:
Attribution
Author:David T. Moran, MD
Date Written:
Editor:Crist, Gayle P., MS, BA
Edit Date:06/25/02
Reviewer:Kathleen A. MacNaughton, RN, BSN
Date Reviewed:06/24/02