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
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
- 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.
March 12, 2015
- American Diabetes Association. Classification and diagnosis of diabetes. Diabetes Care. 2015;38(suppl):S8.
- Diagnosing diabetes and learning about prediabetes. American Diabetes Association. http://www.diabetes.org/diabetes-basics/diagnosis. Accessed Jan. 27, 2015.
- Glucose tests. Lab Tests Online. http://labtestsonline.org/understanding/analytes/glucose/tab/test. Accessed Jan. 27, 2015.
- American College of Obstetricians and Gynecologists (ACOG) Committee on Practice Bulletin – Obstetrics. ACOG Practice Bulletin No. 137. Gestational diabetes mellitus. Obstetrics and Gynecology. 2013;122:406.
- Coustan DR, et al. Diabetes mellitus in pregnancy: Screening and diagnosis. http://www.uptodate.com/home. Accessed Jan. 27, 2015.
- AskMayoExpert. What tests are needed to confirm the diagnosis of gestational diabetes mellitus during pregnancy? Rochester, Minn.: Mayo Foundation for Medical Education and Research; 2014.