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 healthcare provider
will draw your blood to check your blood glucose levels and determine your risk for
diabetes, prediabetes, or gestational diabetes.
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 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
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,
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 healthcare
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.
If you have an abnormal blood glucose, your healthcare provider may recommend behavioral
counseling 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.
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 healthcare provider's instructions on how to prepare for a glucose tolerance
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. 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 healthcare provider 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.