A glucose tolerance test is a multistep test to screen for or diagnose diabetes and other conditions. It typically involves drinking a glucose (sugar) liquid and getting blood draws over a certain period of time. You’ll need to follow specific instructions from your healthcare provider to prepare for this test.
A glucose tolerance test (GTT) is a test that checks to see if your body can use and store glucose (sugar) as it should. Healthcare providers most often use this test to diagnose diabetes. But there are other diabetes diagnostic tests, like an A1C test and fasting blood sugar test, and other purposes for a GTT.
Glucose mainly comes from carbohydrates in the food and drinks you consume. It’s your body’s primary source of energy. Your blood carries glucose to all your body’s cells to use for energy. Your body also stores glucose (mainly in your muscles and liver) for future use. The stored version of glucose is called glycogen.
Several bodily processes help keep your blood glucose (sugar) in a healthy range. Insulin, a hormone your pancreas makes, is the most significant contributor to maintaining healthy blood sugar. Issues with the way your body uses glucose typically result in high blood sugar (hyperglycemia). But it can also cause low blood sugar (hypoglycemia) episodes.
There are several variations of the glucose tolerance test. The most common is the oral glucose tolerance test (OGTT).
Needing to do any type of diagnostic test can be anxiety-inducing. Try to remember that information is power. Regardless of the results, you’ll know more about your body than you did before. With that information, you and your healthcare provider can put together a plan to ensure the healthiest path forward.
Your healthcare provider may recommend a glucose tolerance test to help screen for or diagnose any of the following conditions:
It’s important to note that other tests can help diagnose these conditions. Your provider may recommend more than one diagnostic test.
A one-hour glucose challenge test during pregnancy is a routine test to screen for gestational diabetes. If the results are abnormal, your provider will recommend a three-hour OGTT to officially diagnose or rule out the condition.
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Because there are several variations of the glucose tolerance test, the specific steps you need to take to prepare for it may vary. In general, you’ll likely need to:
Your healthcare provider will give you specific instructions based on your unique situation and the type of GTT you’re doing. Be sure to follow them, and don’t hesitate to ask questions.
The specific steps of a glucose tolerance test can vary depending on the type. As an example of the steps, we’ll use the most common type: the two-hour oral glucose tolerance test (OGTT). In most cases, you’ll start the test in the morning — usually between 7:00 and 9:00 a.m. You’ll stay at the hospital or medical building until the end of the test, so be sure to bring something (like a book) to entertain yourself while you wait.
The test typically involves the following steps:
The testing process varies for certain types of GTTs. For example:
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After the glucose tolerance test, you should be able to go about your regular activities. You can also resume eating after the test. You may want to bring a snack with you to have after the test. Your healthcare provider will give you further instructions if necessary and let you know when to expect the results.
In most cases, you should expect the results of a glucose tolerance test within a few business days. The amount of time it takes varies from lab to lab.
Glucose tolerance tests are generally safe. Most people don’t have any side effects from the drink. Some people say it makes them feel nauseated or dizzy, but serious side effects are rare.
Blood tests (draws) are a very common and essential part of medical testing. There’s very little risk to having blood tests. You may have slight tenderness or a bruise at the site of the blood draws, but this usually resolves quickly.
Your glucose tolerance test results will include the blood glucose level value for each of the blood draws. Labs in the U.S. typically measure blood glucose in milligrams per deciliter (mg/dL).
The ranges laboratories and healthcare providers use to diagnose conditions related to blood sugar issues (like diabetes) may vary slightly. Always refer to the reference range the report gives.
In general, providers use the following ranges for the two-hour glucose tolerance test:
For gestational diabetes and other conditions, the ranges and timing intervals may be different. For example, the GTT for an acromegaly diagnosis assesses the change in growth hormone levels. Your provider will go over the results with you.
If your glucose tolerance test results are abnormal, your healthcare provider will explain the next steps. It can be stressful to see an abnormal result. But know that providers generally don’t make a diagnosis of prediabetes or diabetes after one abnormal OGTT.
You’ll likely need to repeat the test on another day shortly after, or your provider will use another diagnostic test, like an A1C. In any case, your provider will be by your side throughout the diagnostic process. Don’t hesitate to ask questions or voice your concerns.
A note from Cleveland Clinic
Seeing an abnormal test result can be stressful. Know that having an abnormal glucose tolerance test result doesn’t necessarily mean you have diabetes. If your results show you have elevated blood sugar levels, your healthcare provider will likely have you take further tests to determine a diagnosis. Don’t hesitate to ask your provider questions. They’re available to help you.
Last reviewed on 08/16/2024.
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Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy