The glucose tolerance test, also known as the oral glucose tolerance test, measures your body's response to sugar (glucose). The glucose tolerance test can be used to screen for type 2 diabetes. More commonly, a modified version of the glucose tolerance test is used to diagnose gestational diabetes — a type of diabetes that develops during pregnancy.

The glucose tolerance test identifies abnormalities in the way your body handles glucose after a meal — often before your fasting blood glucose level becomes abnormal.

It's important to eat and drink normally in the days leading up to the glucose tolerance test. Let your doctor know if you're ill or taking any medications, as these factors can affect the results of your test.

For eight hours before the test, you won't be able to eat or drink anything. You might want to fast overnight and schedule the test for early the following morning.

The glucose tolerance test is done in several steps. When you arrive at your doctor's office or lab, a member of your health care team will take a sample of blood from a vein in your arm. This blood sample will be used to measure your fasting blood glucose level.

Type 2 diabetes

If you're being tested for type 2 diabetes:

  • You'll drink about 8 ounces (237 milliliters) of a syrupy glucose solution containing 2.6 ounces (75 grams) of sugar
  • Two hours later, your blood glucose level will be measured again

Gestational diabetes

The American Congress of Obstetricians and Gynecologists recommends performing a one-hour blood glucose challenge test to screen for gestational diabetes in low-risk pregnant women between 24 and 28 weeks of pregnancy.

Your doctor may recommend earlier screening if you're at increased risk of developing gestational diabetes. Risk factors may include:

  • Gestational diabetes in an earlier pregnancy
  • Family history of diabetes
  • Obesity
  • Having a medical condition associated with the development of diabetes, such as metabolic syndrome or polycystic ovary syndrome

If your doctor determines you're at risk or you have a suspicious value on the one-hour test, you may be advised to take a three-hour glucose tolerance test.

For the three-hour test:

  • You will be asked to come to the test fasting — not having had anything to eat or drink for the previous 8 hours. A fasting blood sugar will be obtained.
  • You'll drink about 8 ounces (237 milliliters) of a glucose solution containing 3.5 ounces (100 grams) of sugar.
  • Your blood glucose level will be tested again one, two and three hours after you drink the solution.

After drinking the glucose solution, you'll likely need to remain in the doctor's office or lab while you're waiting for your blood glucose level to be tested. After the glucose tolerance test, you can return to your usual activities immediately.

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

Type 2 diabetes

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

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

If the results of your glucose tolerance test indicate type 2 diabetes, your doctor may repeat the test on another day or use another blood test to confirm the diagnosis. Various factors can affect the accuracy of the glucose tolerance test, including illness, activity level and certain medications.

Gestational diabetes

If you're being tested for gestational diabetes, your doctor will consider the results of each blood glucose test.

At Mayo Clinic, if your blood glucose level is higher than 140 mg/dL (7.8 mmol/L) after the one-hour test, your doctor will recommend the three-hour test. If your blood glucose level is higher than 190 mg/dL (10.6 mmol/L) after the one-hour test, you'll be diagnosed with gestational diabetes.

For the three-hour test:

  • A normal fasting blood glucose level is lower than 95 mg/dL (5.3 mmol/L).
  • One hour after drinking the glucose solution, a normal blood glucose level is lower than 180 mg/dL (10 mmol/L).
  • Two hours after drinking the glucose solution, a normal blood glucose level is lower than 155 mg/dL (8.6 mmol/L).
  • Three hours after drinking the glucose solution, a normal blood glucose level is lower than 140 mg/dL (7.8 mmol/L).

If one of the tests is higher than normal, you'll likely need to test again in four weeks. If two or more of the tests are higher than normal, you'll be diagnosed with gestational diabetes.

If you're diagnosed with gestational diabetes, you can prevent complications by carefully managing your blood glucose level throughout the rest of your pregnancy.

March 12, 2015