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Glucose Tolerance Test

Does this test have other names?

Oral glucose tolerance test, OGTT

What is this test?

An oral glucose tolerance test (OGTT) is used to screen for diabetes or prediabetes. To start the test, your healthcare provider will draw your blood to check your blood glucose level. Then you will drink a liquid rich in glucose, or sugar. Your healthcare provider will draw your blood every hour for the next 2 to 3 hours to check your blood glucose. This will help determine your risk for diabetes, prediabetes, or gestational diabetes. It will also help diagnose diabetes.

Why do I need this test?

If you have symptoms of or risk factors for diabetes, your healthcare provider may order an OGTT. Symptoms of diabetes include increased thirst, increased urination, unexplained weight loss, blurred vision, tiredness, and sores that don't heal. Risk factors for diabetes include overweight or obese, physical inactivity, high blood pressure, high cholesterol, or a family history of diabetes. The U.S. Preventive Services Task Force recommends that adults ages 40 to 70 who are obese or overweight have their blood glucose checked at least every 3 years as long as their results are normal. All adults should be tested for diabetes every 3 years beginning at age 45, no matter what their weight. 

The test is a useful first step in diagnosing prediabetes, diabetes, or gestational diabetes.

What other tests might I have along with this test?

Other tests that are used to diagnose diabetes or monitor blood glucose include blood glucose testing and A1C blood tests. Because heart health is so closely tied with diabetes, regular checks of blood pressure, cholesterol, and triglycerides are important, too.

What do my test results mean?

Lab test results may be affected by many things, including the method the lab uses to do the test. If your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your healthcare provider.

This test can be done several ways. For one of the more common types of glucose tolerance tests, a normal glucose level is less than 140 milligrams per deciliter (mg/dL) 2 hours after you drink the glucose-infused fluid. If your level is between 140 and 199 mg/dL, you might have impaired glucose tolerance or prediabetes. If it's 200 mg/dL or above, you might have diabetes, and more testing may be needed. During pregnancy, a result above 153 mg/dL after 2 hours means you may have gestational diabetes.

If you have an abnormal blood glucose, your healthcare provider may recommend medicines, as well as lifestyle changes to help you eat better and get more exercise.

How is this test done?

At a medical facility or lab, a healthcare provider will get you ready for the test by putting an IV called a heparin lock into a vein in your arm. This allows the staff to take multiple blood samples without repeatedly sticking you with a needle.

For one of the more common types of glucose tolerance tests, the first blood sample will be drawn and your blood glucose level will be checked. Then you will be asked to drink a liquid containing glucose, which is about 75 grams of sugar dissolved in water.

After that, your blood will be drawn every hour for the next 2 to 3 hours. Each sample will be checked to measure levels of glucose.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

After drinking the sweet liquid used for this test, you may feel nauseous or get a stomachache or headache. These side effects should go away during the test.

What might affect my test results?

A number of factors, mainly diet and exercise, can affect blood glucose levels. Carefully follow your healthcare provider's instructions on how to prepare for a glucose tolerance test.

How do I get ready for this test?

You will be instructed to not eat or drink anything except water from midnight the night before the test until the time of the test. Don't smoke, chew gum, or exercise other than light walking the day before and the morning of the test.

Be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any street drugs you may use.

Medical Reviewers:

  • Haldeman-Englert, Chad, MD
  • Holloway, Beth Greenblatt, RN, M.Ed.