Glucose Tolerance Test

Written by Tricia Kinman
Medically Reviewed by George Krucik, MD

Glucose Tolerance Test

A glucose tolerance test measures how well your body is able to break down glucose, or sugar. Those who suffer from diabetes (type 1) have trouble processing glucose because the body is not able to make an adequate supply of insulin. This test is also used to diagnose the presence of gestational diabetes and type 2 diabetes. Gestational diabetes is when a pregnant woman who is not a diabetic, has high blood sugar levels as a result of the pregnancy. According to the American Diabetes Association, gestational diabetes occurs in 18 percent of pregnancies, usually around the second trimester (ADA).

Type 2 diabetes develops during adulthood, unlike type 1 diabetes, which occurs during childhood. Type 2 diabetes is the most common form of diabetes. In this type, your body either doesn’t produce enough insulin, the hormone in the body that metabolizes sugar, or the cells in your body are resistant to insulin.

There are two kinds of glucose tolerance tests: the oral glucose tolerance test (OGTT), which is more common, and the intravenous glucose tolerance test (IGTT).This test is also called fasting blood sugar test (FBS).

Who Needs a Glucose Tolerance Test?

Generally most healthcare providers recommend that all pregnant women be screened for gestational diabetes. Experts recommend this test to pregnant women who are between 24 and 28 weeks of pregnancy (Lowdermilk). This test is also recommended for anyone suspected of developing adult diabetes.

Preparing for a Glucose Tolerance Test

There are some things you need to do to prepare for the glucose tolerance test:

  • Continue to eat a normal diet in the days leading up to test.
  • Consult with your physician about any medications that you are currently taking. Some medications, such as beta-blockers and antidepressants, can interfere with the results (Pagana).
  • Abstain from food for at least eight hours before the scheduled test. You may drink water, but avoid other beverages, as these can interfere with the results.
  • Avoid going to the bathroom just before the procedure, as you may be asked for a urine sample.
  • Bring something to read or an activity to keep you busy while you wait.

You may be scheduled for a one-, two-, or three-hour test. Normally, the one-hour test is a screening for diabetes. Most women who have low risk factors for developing diabetes will take this test to rule out the condition.

If you have risk factors for gestational or type 2 diabetes, your doctor will probably have you take the two- or three-hour test.

During the Test

The test can take place in your physician’s office or a local laboratory. When you arrive, they will take a small sample of blood and a urine sample.

After giving a blood sample, you will be asked to drink an extremely sweet and concentrated solution of glucose within a given amount of time (usually five minutes). After this, you will be asked to sit in the waiting area until your glucose levels will be tested again. If you are taking the 50-gram, or one-hour test, they will take your blood sample after one hour. If you are taking the 75-gram, or two-hour test, they will take a blood sample every hour for two hours. If you are taking the 100-gram, or three-hour test, they will take a blood sample every hour for three hours.

By taking several samples of your blood as your body processes the sugary drink, your healthcare provider will be able to tell how quickly your body can process sugar.

Risks of Glucose Tolerance Test

There are no associated risks to you or your baby when taking the glucose tolerance test. There is always a slight risk of infection when the skin barrier is broken, so watch for signs of infection, such as redness and swelling around the puncture site, and fever.

You may also feel faint or dizzy from not eating. It’s a good idea to eat after the test.

Results of Glucose Tolerance Test

If your test shows that your glucose levels are higher than normal, you may be asked to test again on a different day. Higher-than-normal levels of glucose may mean you have prediabetes, diabetes, or gestational diabetes (MayoClinic). Results may vary, but generally:

Normal glucose levels are:

  • 50-gram (one-hour test): equal or less than 140 mg/dL
  • 75-gram (two-hour test): fasting levels 60-100 mg/dL ; one-hour levels less than 200 mg/dL; two-hours less than 140 mg/dL
  • 100-gram test: fasting levels less than 95 mg/dL ; one-hour levels less than 180 mg/dL; two hours less than 155 mg/dL; three hours, lesson than 140 mg/dL

Treatment for Positive Glucose Tolerance Test

Based on the results, your doctor will recommend a course of treatment. If you have abnormal levels of glucose, you may be able to manage diabetes with diet and exercise. You may be referred to a nutritionist. In some cases, insulin is prescribed. If you are pregnant and have gestational diabetes, it is vital that you manage and monitor your blood glucose levels, as it can affect your pregnancy.

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