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
sugar (glucose) level. Then you will drink a liquid with a lot of glucose. Your healthcare
provider will draw your blood every hour for the next 2 to 3 hours to check your blood
glucose. This will help assess your risk for diabetes, prediabetes, or gestational
diabetes. It will also help diagnose diabetes.
Why do I need this test?
The test is a first step in diagnosing prediabetes, diabetes, or gestational diabetes.
You may need this test if you have symptoms of diabetes. These include:
Unexplained weight loss
Sores that don't heal
You may also need this test if you have risk factors for diabetes. These include:
The U.S. Preventive Services Task Force advises that adults ages 40 to 70 who are
obese or overweight have their blood glucose checked at least every 3 years. All adults
should be tested for diabetes every 3 years starting at age 45, no matter what their
weight. Adults with diabetes should have their blood tested much more often.
What other tests might I have along with this test?
You may need tests to diagnose diabetes or watch your blood glucose levels. These
Blood glucose testing
A1C blood test
Heart health is closely tied with diabetes. Because of this, you will need regular
tests of your:
What do my test results mean?
Test results may vary depending on your age, gender, health history, the method used
for the test, and other things. Your test results may not mean you have a problem.
Ask your healthcare provider what your test results mean for you.
This test can be done several ways. For a common type of glucose tolerance test, a
normal glucose level is less than 140 milligrams per deciliter (mg/dL) 2 hours after
you drink the glucose fluid. If your level is 140 to 199 mg/dL, you might have impaired
glucose tolerance. This is also known as prediabetes. If it's 200 mg/dL or above,
you might have diabetes, and need more tests. During pregnancy, a result above 153
mg/dL after 2 hours means you may have gestational diabetes.
If you have high blood glucose, your healthcare provider may advise lifestyle. These
will include changing your diet and getting more exercise. Your healthcare provider
may also prescribe medicine to help manage your blood sugar levels.
How is this test done?
This test is done at a medical facility or lab. A healthcare provider will put an
IV called a heparin lock into a vein in your arm. This allows the staff to take multiple
blood samples more easily.
For a common type of glucose tolerance test, the first blood sample will be drawn
and your blood glucose level will be checked. Then you will be asked to drink a liquid
that has a lot of glucose. It will contain 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 the levels of glucose.
Does this test pose any risks?
Having a blood test with a needle carries some risks. These include bleeding, infection,
bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may
feel a slight sting or pain. Afterward, the site may be sore.
After drinking the sweet liquid used for this test, you may feel nauseated. You may
have a stomachache or headache. These side effects should go away after the test.
What might affect my test results?
A number of factors can affect blood glucose levels. Follow your healthcare provider's
instructions on how to prepare for the test.
How do I get ready for this test?
Don't eat or drink anything except water after midnight before the test. Don't smoke,
chew gum, or exercise other than light walking the day before and the morning of the
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 illegal
drugs you may use.