What is an oral glucose tolerance test (OGTT)?

An oral glucose tolerance test (OGTT) measures how well your body uses sugar (glucose). It can be used to find prediabetes and diabetes. It's done most often to screen for diabetes during pregnancy (gestational diabetes).

Why is this test done?

An OGTT may be done to:

• Check for prediabetes and diabetes.
• Check pregnant women for gestational diabetes.

How do you prepare for the test?

• Tell your doctor about all the prescription and non-prescription 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 is the test 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 50, 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 does having an oral glucose tolerance test (OGTT) feel?

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.

What happens after the test?

• You will probably be able to go home right away, depending on the reason for the test.
• You can go back to your usual activities right away.