Description of the test

The glucose tolerance test measures the body's ability to use glucose. Glucose is a type of sugar found in the blood. Glucose levels, also called blood sugar levels, are regulated by the hormone insulin. When insulin works properly, blood sugar levels in the blood remain controlled. If insulin does not work properly or the body does not produce enough insulin, you may have diabetes. The glucose tolerance test helps doctors determine if you have diabetes. It can also identify hypoglycemia (low blood sugar).

Testing can be done in a doctor's office, clinic, or hospital.

How often should the test be performed?

The test is routinely used during pregnancy to determine whether a woman has developed gestational diabetes (a temporary form of diabetes caused by pregnancy). It can also be used on a regular basis to screen people who are at risk of diabetes. In special circumstances, it is used to check for hypoglycemia, or low blood sugar.

Why is this test performed? This test helps the doctor determine if you have diabetes. It also helps determine if you have another condition that affects blood glucose levels (e.g., Cushing's syndrome, celiac disease, cystic fibrosis, acromegaly, pheochromocytoma, hemochromatosis, Addison's disease, hypothyroidism, hypoglycemia, or cirrhosis).

Are there any risks and precautions?

Although the glucose tolerance test is considered safe, it does have some risk of side effects or complications. Though rare, the side effects or complications may include:

• infection, if the area is not properly sterilized before the sample is taken
• excess bleeding from the area that was punctured
• bruising where the needle was inserted

You will be monitored throughout the test to make sure that your blood glucose level does not drop too low. You may experience weakness, sweating, anxiety, hunger, or other symptoms if this happens. Be sure to tell the person performing the test if you experience any of these symptoms.

People with bleeding disorders or anyone taking medication that reduces the ability of the blood to clot (e.g., warfarin, ASA, or other anticoagulants) should tell the technician before the samples of blood are taken. These conditions and medications may require special attention at the time of testing.

What happens during the test?

If the test is being done for a pregnant woman, she will be asked to drink a sweet drink containing glucose. Approximately one hour after drinking the liquid, a sample of blood will be taken. For everyone else, a sample of blood will be taken when they arrive.

Blood is collected into one or more small vials labeled with your information.

The technician should wear fresh latex gloves and use a new, single-use needle for each procedure.

To draw the blood sample, a technician or nurse wraps a wide elastic band around your upper arm to help locate the vein. Once the vein is located, usually on the inside of your elbow, the area is disinfected using alcohol swabs. The technician inserts a needle into a vein and pushes a vial onto the needle to collect the blood. The elastic band is then removed and after the blood is collected, the needle is removed and discarded. You will have to hold a small piece of cotton over the area until bleeding stops. A small bandage will be applied to hold the cotton in place.

From this sample, the initial level of glucose in your blood is determined.

Next, you will be asked to drink a sweet beverage containing glucose. Depending on which test is being done, a health care professional will take one or more samples of blood as often as every hour, for up to five hours. The samples are then analyzed at a laboratory.

The test is almost painless for most people, although you may feel a slight pinch when the needle is inserted.

How should I prepare for this test?

Pregnant women do not need to do any preparation before the test.

If you are not pregnant, you must:

• eat a specific amount of carbohydrates for three days before the test. Your doctor will advise you on what you should be eating during this time.
• fast (i.e., eat and drink nothing) for 8 to 14 hours before the test. Your doctor will tell you how long you need to fast.
• avoid smoking and exercising for 8 to 14 hours before the test.

Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications that you are taking. Also tell them about any medication allergies and medical conditions that you may have.

Ask your doctor or pharmacist whether you need to stop taking any of your medications before the test.

What can I expect after the test?

Some people experience slight bruising or tenderness where the needle entered the vein. Keep pressure on the area after the needle is removed to prevent bruising.

Results

Values for the level of blood glucose after the test are considered either lower than normal, normal, or higher than normal. Several conditions can cause higher- or lower-than-normal blood sugar levels. For example, if you are pregnant and your results are higher than normal, you may have gestational diabetes. If you are not pregnant and your results are higher than normal, you may have diabetes. If your results are lower than normal, you may have hypoglycemia.

Your doctor will interpret the results of this test in combination with other symptoms you may have.