OverviewThe 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. Show
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Why it's doneThe glucose tolerance test identifies abnormalities in the way your body handles glucose after a meal — often before your fasting blood glucose level becomes abnormal. RisksThe risks associated with obtaining a blood sample are slight. After your blood is drawn, you may experience bruising or bleeding. You may also feel dizzy or lightheaded. In some cases, infection after your procedure is possible. Sign up for free, and stay up to date on research
advancements, health tips and current health topics, like COVID-19, plus expertise on managing health. To provide you with the most relevant and helpful information, and understand which information is beneficial, we may combine your email and website usage information with other information we have about you. If you are a Mayo Clinic patient, this could include protected health information. If we combine this information with your protected health information, we will treat all of that information as protected health information and will only use or disclose that information as set forth in our notice of privacy practices. You may opt-out of email communications at any time by clicking on the unsubscribe link in the e-mail. How you prepareFood and medicationsIt'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. What you can expectBefore the procedureFor 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. During the procedureThe 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 diabetesIf you're being tested for type 2 diabetes:
Gestational diabetesThe American College 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:
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:
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 procedureAfter the glucose tolerance test, you can return to your usual activities immediately. ResultsResults of the oral glucose tolerance test are given in milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L). Type 2 diabetesIf you're being tested for type 2 diabetes, two hours after drinking the glucose solution:
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 diabetesIf 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:
If one of the results is higher than normal, you'll likely need to test again in four weeks. If two or more of the results 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. Clinical trialsExplore Mayo Clinic studies of tests and procedures to help prevent, detect, treat or manage conditions. When should you stop eating sugar before glucose test?DO NOT eat or drink anything (other than sips of water) for 8 to 14 hours before your test. (You also cannot eat during the test.) You will be asked to drink a liquid that contains glucose (75 g).
Can what you eat the day before affect your glucose test?Glucose tests come with specific instructions — such as fasting — and it's important to follow them. Changes to your diet a day or two before a glucose test won't make a huge difference to your results.
What should I not do the night before my glucose test?DO NOT eat or drink anything except WATER for at least 8 hours before the test. You may drink plain water ONLY. Do NOT drink coffee, tea, soda (regular or diet) or any other beverages. Do NOT smoke, chew gum (regular or sugar-free) or exercise.
What should you eat the day before glucose test?Here is the best method of preparation: Eat a balanced diet that contains at least 150 grams of carbohydrates per day for 3 days before the test. Fruits, breads, cereal, rice, crackers, and starchy vegetables such as potatoes, beans and corn are good sources of carbohydrates.
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