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). The test can be done on its own or as the second test in a two-part screening for pregnant women.
Why It Is Done
An OGTT may be done to:
- Check for prediabetes and diabetes.
- Check pregnant women for gestational diabetes.
How To Prepare
- Tell your doctor about all the prescription and nonprescription medicines you are taking. You may be told to stop taking certain medicines before the test.
- Do not eat, drink, smoke, or do strenuous exercise for at least 8 hours before your first blood sample is taken. (You can drink water before the test.)
How It Is Done
- A blood sample is taken when you arrive for the test. This is your fasting blood glucose value. It will be compared to other glucose values in your blood.
- You will drink a small cup of very sweet liquid that contains 75 or 100 grams of glucose.
- You will have more blood tests over 1 to 3 hours.
- Since activity can affect test results, you will be asked to sit quietly during the entire test. Do not eat during the test. You may drink water during this time.
How long the test takes
The test may take up to 3 hours, plus the time it takes for the doctor to meet with you.
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.
When a blood sample is taken, you may feel nothing at all from the needle. Or you might 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.
There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.
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.
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.
Current as of: April 13, 2022
Author: Healthwise Staff
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