Glucose Tolerance Test: Purpose, Procedure & Risks

Glucose Tolerance Test

Glucose tolerance test


  1. A glucose tolerance test is used to help diagnose gestational diabetes and type 2 diabetes.
  2. There are three versions of the oral glucose tolerance test: the 1-hour test, the 2-hour test, and the 3-hour test.
  3. In the eight hours leading up to the test, you should not eat food or drink any beverages aside from water.

A glucose tolerance test measures how well your body can handle sugar. During this test, you drink a certain amount of sugar, and then your blood sugar levels are monitored. Doctors use this test to help diagnose all types of diabetes, though it’s most commonly used with type 2 diabetes and gestational diabetes.

People with diabetes are unable to get sugar from the blood and into the cells due to either a lack of insulin, the cell’s being resistant to insulin, or both. Type 2 diabetes is the most common form of diabetes. This type of diabetes most often occurs in adults, although it’s now appearing in people of all ages. It’s common in people who are overweight, obese, or sedentary. It’s also common in people who have a family history of type 2 diabetes. To test for type 2 diabetes, a 2-hour glucose tolerance test is used.

Gestational diabetes is when a pregnant woman with no prior history of diabetes develops high blood sugar levels as a result of the pregnancy. According to the American Diabetes Association (ADA), gestational diabetes occurs in 9.2 percent of pregnancies, usually around the second trimester. To test for gestational diabetes, either a 1-hour or 2-hour test can be used. If needed, doctors may perform a 3-hour test in addition to the 1-hour test.

Who needs a glucose tolerance test?

If you have signs and symptoms of diabetes, or you have an abnormal glucose reading on a general lab test, your doctor may order a 2-hour glucose tolerance test to test for type 2 diabetes.

Healthcare providers recommend that all pregnant women be screened for gestational diabetes. This test is typically done between 24 and 28 weeks of pregnancy, according to the American College of Obstetricians and Gynecologists.

Read more: Understanding glucose levels »

Preparing for a glucose tolerance test

You can prepare for a glucose tolerance test by taking these steps:

  • Continue to eat a normal diet in the days leading up to the test.
  • Consult with your doctor about any medications you’re taking. Some medications, such as beta-blockers and antidepressants, can interfere with the results.
  • Avoid eating food for at least eight hours before the test. You may drink water, but avoid other beverages, including those with caffeine, 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 1-, 2-, or 3-hour test. Normally, the 1-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.

During the test

The test can take place in your doctor’s office or a laboratory. When you arrive, your healthcare provider 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 sugar within a given amount of time (usually five minutes). You will then sit in a waiting area until your glucose levels are tested again.

  • If you take the 50-gram 1-hour test, your healthcare provider will take a blood sample after one hour.
  • If you take the 75-gram 2-hour test, your healthcare provider will take a blood sample every hour for two hours.
  • If you take the 100-gram 3-hour test, your healthcare provider will take a blood sample every hour for three hours.

By taking several samples of your blood as your body processes the sugar from the drink, your healthcare provider will be able to tell how well your body can mange a sugar load.

Risks of glucose tolerance test

If you’re pregnant, there are no associated risks to you or your baby when taking the glucose tolerance test.

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

Results of glucose tolerance test

For type 2 diabetes, if your test shows that your glucose levels are higher than normal, you may be asked to test again on a different day. According to the Mayo Clinic, higher-than-normal levels of glucose may mean you have prediabetes, type 1 or 2 diabetes, or gestational diabetes. This depends on your blood sugar level and other symptoms.

The oral glucose test measures your blood glucose level after a period of fasting. This is known as your fasting level. The results are given in milligrams per deciliter (mg/dL).

In general, your doctor will diagnose you with type 2 diabetes if your fasting levels are over 126 mg/dL, or your 2-hour values are greater than or equal to 200 mg/dL. They may diagnose you with prediabetes if your fasting levels are between 100-125 mg/dL, or your 2-hour values are between 140-199 mg/dL.

As mentioned above, your doctor will likely confirm your diagnosis by repeating the test on a different day. They may also use another type of blood test.

According to the ADA 2016 guidelines, the following are diagnostic for gestational diabetes with the 2-hour oral glucose tolerance test:

  • fasting levels greater than or equal to 92 mg/dL
  • 1-hour values of 180 mg/dL or greater
  • 2-hour values of 153 mg/dL or greater

If screening with a 1-hour test, results of 140 mg/dL or greater will lead to a 3-hour test. The following are diagnostic for gestational diabetes with the 3-hour oral glucose tolerance test:

  • fasting levels equal to or greater than 95 mg/dL
  • 1-hour values of 180 mg/dL or greater
  • 2-hour values of 155 mg/dL or greater
  • 3-hour values of 140 mg/dl or greater

Treatment for positive glucose tolerance test

Based on the results, your doctor will recommend a course of treatment.

For type 2 and gestational diabetes, you may be able to manage diabetes with diet and exercise. Your doctor may refer you to a dietitian. In some cases, your doctor may prescribe medication.

If you’re pregnant and have gestational diabetes, it’s important that you manage and monitor your blood glucose levels, as it can affect your pregnancy. If you have type 2 diabetes, good blood sugar management can help you avoid long-term complications.

Preventing gestational diabetes

You Asked, We Answered

  • What can I do to help prevent gestational diabetes?
  • Insulin resistance is a normal part of pregnancy. It’s the body’s way to ensure that the fetus has enough fuel for growth. Because of this, it’s difficult to prevent gestational diabetes. However, there are ways you can lower your risk. If you’re overweight before your pregnancy, consider losing weight before becoming pregnant. You should not lose weight during pregnancy. Regular exercise can help as well, both before and during pregnancy. If you have not been exercising, check with your doctor before starting during pregnancy. Choose high-fiber whole carbohydrates sources mixed with protein and fats to help keep blood sugar stable. 

    - Peggy Pletcher, MS, RD, LD, CDE

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