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 only 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, however, 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 by pricking your fingertip or inserting a needle into 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 test first to screen for gestational diabetes in low-risk patients. If your doctor determines you're at risk or you have a suspicious value on the one-hour test, you may take the longer blood glucose test. For this 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 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. If two or more of the tests are higher than normal, you'll be diagnosed with gestational diabetes.

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

Apr. 17, 2013