Glucose Tolerance Test (GTT)

What is a glucose tolerance 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.

When would I need a glucose tolerance test?

Your healthcare provider may recommend a glucose tolerance test to help screen for or diagnose any of the following conditions:

• Diabetes (usually prediabetes, Type 2 diabetes or gestational diabetes).
• Insulin resistance.
• Reactive hypoglycemia (low blood sugar).
• Acromegaly.
• Rare carbohydrate metabolism disorders.

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.

How do I prepare for a glucose tolerance test?

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:

• Tell your provider about all the medications and supplements you take. Some medications can affect the results of the test. Your provider may tell you to stop taking certain medications before the test.
• Eat as you normally do for at least three days before the test. You should consume at least 150 grams of carbohydrates each day.
• Maintain your typical activity level for at least three days before the test. Don’t exercise excessively before the test.
• Fast (don’t eat or drink anything except water) right before the test — usually at least eight hours.
• Not eat or smoke cigarettes during the test.

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.

What happens during a glucose tolerance test?

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:

• A provider (typically a phlebotomist) will take a blood sample from your vein. This is to measure your fasting blood glucose.
• A provider will give you a glucose liquid to drink. It’s usually 75 grams of glucose dissolved in 250 to 300 mL of water. The provider will instruct you to drink it all within five minutes. The dose is different for children and pregnant people. The glucose drink tastes like extra sweet Gatorade® or flat soda, which may taste unpleasant for some people.
• A provider will take a blood sample from your vein two hours after drinking the glucose to check your blood glucose. Depending on the reason for the test, they may draw your blood several times every 30 to 60 minutes after you drink the solution.

Other variations of the glucose tolerance test

The testing process varies for certain types of GTTs. For example:

• One-hour glucose tolerance test: This is typically a glucose challenge test to screen for gestational diabetes during pregnancy. You drink a liquid with 50 grams of glucose and then a provider draws your blood to check your blood sugar one hour later. You typically don’t need to fast for this test.
• Three-hour glucose tolerance test: You’ll have blood draws before you drink the glucose liquid (fasting), and at one, two and three hours after drinking it.
• Glucose tolerance test for acromegaly: During this test, your provider will draw blood samples to check your human growth hormone (HGH) level (not your blood sugar). After you drink the glucose liquid, they’ll likely draw your blood every 30 minutes for two hours at 30-, 60-, 90- and 120-minute intervals.

What should I expect after a glucose tolerance test?

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.

When should I know the results of a glucose tolerance test?

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.

What are the possible side effects of a glucose tolerance test?

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.

What do the results of a glucose tolerance test mean?

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:

• Normal: Under 140 mg/dL.
• Prediabetes: 140 to 199 mg/dL.
• Diabetes: At or over 200 mg/dL.

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 the results are abnormal, what are the next steps?

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.