Oral Glucose Tolerance Test

An oral glucose tolerance test (OGTT) is a blood test that checks your blood sugar levels before and after you drink a sweet drink that contains glucose (sugar). The test, which is also known as the glucose tolerance test, measures how your body responds to sugar.1

Why is the oral glucose tolerance test important?

Blood sugar testing gives you and your doctor important clues about your health. The OGTT is used to see how your body moves sugar from your blood into your muscles and fat.1

The OGTT is most often used to diagnose gestational diabetes, a form of diabetes that can affect pregnant women. It can also be used to diagnose type 2 diabetes.1

Why do pregnant women need to take the oral glucose tolerance test?

Some women develop high blood sugar while they are pregnant. This is known as gestational diabetes. In most cases, it is a temporary form of diabetes in which the body does not make enough insulin during pregnancy. About 2 to 5 percent of pregnant women develop gestational diabetes, though women who have risk factors for type 2 diabetes are at higher risk.3

If you have gestational diabetes that is not diagnosed or treated, it can cause complications for both you and your baby, including:3

  • Premature (early) delivery
  • A baby with a high birth weight
  • Increased chance of cesarean delivery
  • Increased risk of preeclampsia (dangerous high blood pressure during pregnancy)

If gestational diabetes is diagnosed and properly treated, it reduces your risk of pregnancy complications. In most cases, women with gestational diabetes that is diagnosed and managed, have healthy babies and their blood sugar levels return to normal after delivery.3

How is the oral glucose tolerance test done?

You should eat and drink normally in the days before your oral glucose tolerance test. Tell your doctor about any medicines you are taking or if you are sick before taking the test since these can affect the results of your test.1

You should fast (not eat or drink anything besides water) for 8 to 14 hours before your test. Your doctor will give you specific instructions before your test. If possible, schedule your test for early in the morning so you can fast overnight.1

The test is done in a few different steps, which vary slightly depending on if you are being tested for gestational or type 2 diabetes. When you arrive at your doctor’s office or lab, a member of your healthcare team will take a sample of your blood from a vein in your arm. This sample will be used to measure your fasting blood sugar level.1

Oral glucose tolerance test for type 2 diabetes

If your doctor is using the OGTT to test you for type 2 diabetes:1

  • You will drink about 8 ounces of a sweet glucose drink that has 2.6 ounces (75 grams) of sugar
  • 2 hours after finishing the drink, your blood glucose level will be measured again

Oral glucose tolerance test for gestational diabetes

If your doctor is using the OGTT to test you for gestational diabetes, you will probably have to first take the 1-hour test. This is usually done between 24 and 28 weeks of pregnancy. If you are at risk for gestational diabetes, your doctor may perform the OGTT sooner.1

  • You will drink about 8 ounces of a sweet glucose drink that has 2.6 ounces (75 grams) of sugar
  • 1 hour after finishing the drink, your blood glucose level will be measured again

If you are at risk for gestational diabetes or have high blood sugar levels after the 1-hour test, your doctor may have you take the 3-hour OGTT. For that test, you will need to fast and provide another blood sample. After that:1

  • You will drink about 8 ounces of a sweet glucose drink that has 3.5 ounces (100 grams) of sugar
  • Your blood glucose level will be tested 1, 2, and 3 hours after finishing the drink

What do oral glucose tolerance test results mean?

Doctors measure the results of the OGTT differently depending on whether you are being tested for type 2 diabetes or gestational diabetes.

Oral glucose tolerance test results for type 2 diabetes

If your doctor is testing you for type 2 diabetes, they will look at your blood glucose levels 2 hours after you drank the glucose drink.1

  • Normal: A blood glucose level that is lower than 140 mg/dL
  • Prediabetes: A blood glucose level between 140 and 199 mg/dL
  • Diabetes: A blood glucose level of 200 mg/dL or higher

If your test result indicates diabetes, your doctor may have you take the test again on a different day to confirm your diagnosis.1

Oral glucose tolerance test results for gestational diabetes

The American Diabetes Association uses the following OGTT blood glucose levels to diagnose gestational diabetes:4

  • 95 mg/dL or higher after fasting
  • 180 mg/DL or higher 1 hour after the glucose drink
  • 155 mg/dL or higher 2 hours after the glucose drink
  • 140 mg/dL or higher 3 hours after the glucose drink

If 1 of your test results is above normal, your doctor will probably retest you in 4 weeks. If 2 or more of your test results are above normal, you will be diagnosed with gestational diabetes. Talk to your doctor about how you can manage your blood glucose levels throughout your pregnancy.4

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Written by: Heather Morse | Last reviewed: October 2020.