Oral Glucose Tolerance Test (OGTT)

• 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.

Watch

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.

Normal

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 values

High glucose levels may be caused by:

• Diabetes.
• Gestational diabetes.
• Hyperthyroidism.
• Some medicines, such as corticosteroids, Opens dialog, niacin, phenytoin (Dilantin), some diuretics, Opens dialog, and some medicines used to treat high blood pressure, HIV, or AIDS.
• Large amounts of the hormone cortisol in the blood (Cushing's syndrome, Opens dialog).
• Inherited diseases, such as hemochromatosis, Opens dialog.
• Pheochromocytoma, Opens dialog.

Low values

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, Opens dialog).
• Problems with the thyroid gland or an underactive pituitary gland, Opens dialog.
• 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.