The glucose tolerance test measures the body's response to sugar, also called glucose. Another name for this test is the oral glucose tolerance test.

This test can be used to screen for type 2 diabetes or prediabetes before you have symptoms of either condition. Or it can help find out whether diabetes is causing existing symptoms. More often, a version of the test is used to check for diabetes that happens during pregnancy. That condition is called gestational diabetes.

The glucose tolerance test finds problems with the way the body handles sugar after a meal. As you eat, your body breaks down food into sugar. The sugar enters your blood, and the body uses the sugar for energy. But with prediabetes and diabetes, the sugar level in the blood becomes too high.

The risks linked with having a blood sample taken are small. After your blood is taken, you may have bruising or bleeding. You also might feel dizzy or lightheaded. Rarely, an infection can happen after the procedure.

Food and medicines

Eat and drink as you usually would in the days leading up to the glucose tolerance test. Let your healthcare team know if you become sick or if you take any medicines. These factors can affect the results of your test.

Before the procedure

For eight hours before the test, you won't be able to eat or drink anything. This is called fasting. You might schedule the test for early in the morning and fast the night before.

During the procedure

You take the test at your healthcare professional's office, or in a clinic, hospital or lab. The test includes a few steps. First, a member of your healthcare team takes a sample of blood from a vein in your arm. This blood sample is used to measure your fasting blood sugar level.

Type 2 diabetes and prediabetes

Next, if you're being tested for type 2 diabetes or prediabetes:

• You drink a syrupy glucose solution that has 75 grams of sugar.
• One hour later and two hours later, samples of your blood are taken. The blood sugar levels are measured.

This often is called a two-hour glucose tolerance test.

Gestational diabetes

Most pregnant people are screened for gestational diabetes with a different version of the glucose tolerance test. Testing usually is done between 24 and 28 weeks of pregnancy.

Most often, screening includes a two-step process. The first involves the following test. You don't need to fast before this test:

• A sample of your blood is taken, and the blood sugar level is measured.
• Then you drink a glucose solution that has 50 grams of sugar.
• One hour later, another blood sample is taken, and the blood sugar level is checked.

You might hear this called a one-hour glucose tolerance test. If the result shows that your blood sugar is high by a certain amount, your healthcare team will likely suggest another test. This involves taking a longer, three-hour version of the test. For the three-hour test:

• You fast for eight hours first. Afterward, a sample of your blood is taken, and the blood sugar level is measured.
• Then you drink a glucose solution that has 100 grams of sugar.
• Your blood sugar level is tested again one, two and three hours after you drink the solution.

During the three-hour test, you'll likely stay in your healthcare professional's office, or in the clinic or lab. You wait there for your blood sugar levels to be tested.

Less often, a single-step glucose tolerance test is used to screen for gestational diabetes. It's a two-hour test that involves fasting before you drink a glucose solution that has 75 grams of sugar.

Whether you take the one- or two-step glucose tolerance test, the timing of when you get tested could vary. Your healthcare professional may recommend taking it before 24 weeks of pregnancy if you have risk factors for gestational diabetes.

After the procedure

After the glucose tolerance test, you can get back to your usual activities right away. But call your healthcare professional if:

• You have serious pain.
• You have a fever.
• Your skin becomes swollen, changes color or oozes fluid in the area where blood was taken.

Results of the glucose tolerance test are given in milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L).

Type 2 diabetes or prediabetes

If you're being tested for type 2 diabetes or prediabetes, two hours after drinking the glucose solution:

• A healthy blood glucose level is lower than 140 mg/dL (7.8 mmol/L).
• A blood glucose level between 140 and 199 mg/dL (7.8 and 11 mmol/L) is considered prediabetes. If you have prediabetes, you're at risk of developing type 2 diabetes. You're also at risk of heart disease, even if you don't get diabetes.
• A blood glucose level of 200 mg/dL (11.1 mmol/L) or higher suggests diabetes.

If your results suggest diabetes or prediabetes, the test may be repeated on another day. Or another blood test may be used to confirm whether you have diabetes or prediabetes. Various factors can affect how accurate the glucose tolerance test is. These include illness, your activity level and some medicines.

Gestational diabetes

If you're getting screened for gestational diabetes, your healthcare professional considers the results of each blood sugar test.

After the one-hour test, a blood sugar level of 190 milligrams per deciliter (mg/dL), or 10.6 millimoles per liter (mmol/L), or higher indicates gestational diabetes. A blood sugar level below 140 mg/dL (7.8 mmol/L) usually is thought to be within the standard range for the test. But this may vary by clinic or lab.

If your blood sugar level is higher than expected, your healthcare professional likely will recommend the three-hour test.

If you need the follow-up three-hour test:

• A healthy fasting blood sugar level is 95 mg/dL (5.3 mmol/L) or lower.
• One hour after you drink the glucose solution, an expected blood sugar level is 180 mg/dL (10 mmol/L) or lower.
• Two hours after you drink the glucose solution, an expected blood sugar level is 155 mg/dL (8.6 mmol/L) or lower.
• Three hours after you drink the glucose solution, an expected blood sugar level is 140 mg/dL (7.8 mmol/L) or lower.

If one of the results is higher than expected, your healthcare professional might:

• Suggest that you talk about your diet with a nutritionist.
• Advise that you get diabetes screening tests at least every three years.
• Encourage you to lose extra weight after you have your baby.

If two or more of the results are higher than expected, you'll likely be told that you have gestational diabetes.

It's less common, but some pregnant people may receive a standalone two-hour test. You'll likely be told you have gestational diabetes if you have any of the following:

• A fasting blood sugar level of 92 mg/dL (5.1 mmol/L) or higher.
• A blood sugar level of 180 mg/dl (10.0 mmol/L) or higher one hour after you drink the glucose solution.
• A blood sugar level of 153 mg/dL (8.5 mmol/L) or higher two hours after you drink the glucose solution.

If you learn that you have gestational diabetes, your risk of other medical problems is higher. But it's likely that you can prevent them. The key is to control your blood sugar throughout the rest of your pregnancy.