The oral glucose tolerance test (OGTT) measures the body's ability to use a type of sugar, called glucose, that is the body's main source of energy. An OGTT can be used to diagnose prediabetes and diabetes. An OGTT is most commonly done to check for diabetes that occurs with pregnancy (gestational diabetes).
Why It Is Done
The oral glucose tolerance test (OGTT) is done to:
- Check pregnant women for gestational diabetes.
- Diagnose prediabetes and diabetes.
How To Prepare
Glucose tolerance diagnostic test
To prepare for the glucose tolerance diagnostic test:
- Eat a balanced diet that contains at least 150 grams (g) of carbohydrates a day for 3 days before the test. Fruits, breads, cereals, grains, rice, crackers, and starchy vegetables such as potatoes, beans, and corn are good sources of carbohydrates.
- Do not eat, drink, smoke, or exercise strenuously for at least 8 hours before your first blood sample is taken.
- Tell your doctor about all prescription and nonprescription medicines you are taking. You may be instructed to stop taking certain medicines before the test.
The glucose tolerance diagnostic test may take up to 4 hours. Since activity can interfere with test results, you will be asked to sit quietly during the entire test. Do not eat during the test. You may drink only water during this time.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, or how it will be done. To help you understand the importance of this test, fill out the medical test information form .
How It Is Done
Glucose tolerance diagnostic test
On the day of testing, the following steps will be done:
- A blood sample will be collected when you arrive. This is your fasting blood glucose value. It provides a baseline for comparing other glucose values.
- You will be asked to drink a sweet liquid containing a measured amount of glucose. It is best to drink the liquid quickly. For the standard glucose tolerance test, you will drink 75 grams or 100 grams.
- Blood samples will be collected at timed intervals of 1, 2, and sometimes 3 hours after you drink the glucose. Blood samples may also be taken as soon as 30 minutes to more than 3 hours after you drink the glucose.
The health professional taking a sample of your blood will:
- Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
- Clean the needle site with alcohol.
- Put the needle into the vein. More than one needle stick may be needed.
- Attach a tube to the needle to fill it with blood.
- Remove the band from your arm when enough blood is collected.
- Apply a gauze pad or cotton ball over the needle site as the needle is removed.
- Apply pressure to the site and then a bandage.
How It Feels
You may find it hard to drink the extremely sweet glucose liquid. Some people feel sick after drinking the glucose liquid and may vomit. Vomiting may prevent you from completing the test on that day.
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
You may feel faint from having several blood samples taken in one day. But the amount of blood taken will not cause significant blood loss or anemia.
Some people's blood glucose levels drop very low toward the end of the test. But some people feel like their sugar levels are low, when their levels actually are not low. Symptoms of low blood glucose include weakness, hunger, sweating, and feeling nervous or restless. If you develop these symptoms during the test, you may have your sugar level checked quickly with a glucose meter. If your level is very low, the test will be stopped.
There is very little risk of a problem from having blood drawn from a vein.
- You may develop a small bruise at the puncture site. You can reduce the risk of bruising by keeping pressure on the site for several minutes after the needle is withdrawn.
- In rare cases, the vein may become inflamed after the blood sample is taken. This condition is called phlebitis and is usually treated with a warm compress applied several times daily.
The oral glucose tolerance test (OGTT) measures the body's ability to use a type of sugar, called glucose, that is the body's main source of energy.
Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.
High glucose levels may be caused by:
- Gestational diabetes.
- Some medicines, such as corticosteroids, niacin, phenytoin (Dilantin), some diuretics, and some medicines used to treat high blood pressure, HIV, or AIDS.
- Large amounts of the hormone cortisol in the blood (Cushing's syndrome).
- Inherited diseases, such as hemochromatosis.
Low glucose levels may be caused by:
- Certain medicines, such as medicines used to treat diabetes, some blood pressure medicines (such as propranolol), and some medicines for depression (such as isocarboxazid).
- Decreased production of the hormones cortisol and aldosterone (Addison's disease).
- Problems with the thyroid gland or an underactive pituitary gland.
- A tumor or other problems of the pancreas.
- Liver disease.
Many conditions can change blood glucose levels. Your doctor will discuss any significant abnormal results with you in relation to your symptoms and past health.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Recent surgery, illnesses, and infectious diseases.
- Weight loss through dieting.
- Long periods of bed rest (such as from a hospitalization or illness).
What To Think About
When screening for gestational diabetes, a test might be done before the 100-gram oral glucose tolerance test (OGTT). For the first test, you will not need to fast. You will drink 50 grams of glucose. Your blood sample will be taken 1 hour later. If your glucose level is high—for example, over 140 mg/dL (7.8 mmol/L)—you will be asked to take the second test, the 100-gram OGTT.
Even though your gestational diabetes will probably go away after your baby is born, you are at risk for gestational diabetes if you become pregnant again and for type 2 diabetes later in life. You may also have a follow-up glucose tolerance test 4 to 12 weeks after your baby is born or after you stop breastfeeding your baby. If the results of this test are normal, you will still need to be screened for diabetes at least every 3 years.
Other Works Consulted
- Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
- Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.
Current as of: December 19, 2019
Author: Healthwise Staff
Medical Review: E. Gregory Thompson MD - Internal Medicine
Kathleen Romito MD - Family Medicine
Adam Husney MD - Family Medicine
Matthew I. Kim MD - Endocrinology
David C.W. Lau MD, PhD, FRCPC - Endocrinology