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Glucose Tolerance

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, you have a blood glucose test done. Then you will drink a liquid rich in glucose, or sugar. For the next two to three hours, your doctor will draw your blood to check your blood glucose levels and determine your risk for diabetes or prediabetes.

In rare instances, your urine is checked during a glucose tolerance test instead of blood. Urine testing is not as accurate as blood testing and is used only when blood testing can't be done. 

Why do I need this test?

If you have symptoms of or risk factors for diabetes, your health care 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 test is a useful first step in diagnosing prediabetes or 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 an A1C blood test. 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?

A result for a lab test may be affected by many things, including the method the laboratory 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 health care provider.

A normal glucose level is less than 140 milligrams per deciliter (mg/dL) two 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. 

How is this test done?

At a medical facility or lab, a health care 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.

Next, the first blood sample will be drawn and your blood glucose level will be checked. Then you will be asked to drink a sweet liquid containing glucose, which is about 75 grams of sugar dissolved in water.

After that, your blood will be drawn every 30 to 60 minutes for the next two to three hours. Each sample will be checked to measure levels of glucose. Sometimes insulin and growth hormone levels will also be checked. 

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 health care 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 eat a normal diet that contains about 50 percent carbohydrates for three days before the test. Then you will have to not eat or drink anything except water from midnight the night before the test until the time of the test. You should also avoid smoking, chewing gum, and exercise other than light walking the day before and the morning of the test.

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