Why would I need an oral glucose tolerance test?

This test measures the levels of blood sugar (glucose) after ingesting a standard amount of sugar. A high blood glucose level can indicate that you have diabetes. Your physician may want you to have this test to evaluate your sugar metabolism, to check for or to confirm diabetes, or to diagnosis abnormally low blood sugar levels, called hypoglycemia.

An oral glucose tolerance test is the most accurate way to check for diabetes or to determine if a pregnant woman has gestational diabetes, a form of diabetes that develops during pregnancy. During the test, sugar levels in the blood and urine are monitored for three hours after drinking a sugar solution.

Your doctor may have you take this test if you have risk factors for diabetes. These include:

• A family history of diabetes.
• Having had gestational diabetes (which occurs during pregnancy) or previously delivering a baby who weighed 9 pounds or more at birth.
• Being African-American, Hispanic American, Asian American, Native American or a Pacific Islander.
• Obesity.
• 45 years or older.
• High blood pressure, high blood cholesterol or high triglyceride levels.
• Having a condition known as impaired fasting glucose.

If I am pregnant, when should I have this test?

An oral glucose tolerance test is usually done between the 24th and 28th weeks of pregnancy. Women who are considered at high risk (including women who have had gestational diabetes in a previous pregnancy) are often initially screened at an early prenatal visit and, if test results are normal, screened again at 24 to 28 weeks. According to the American Diabetes Association, women younger than 25 who have no risk factors for diabetes do not need screening because they have a very low chance of developing gestational diabetes.

How do I prepare for this test?

Follow a high-carbohydrate diet for three days. Carbohydrate-rich foods include bread, cereals, potatoes, vegetables and fruit. You'll then need to fast for 10 to 16 hours before the test. Do not smoke, drink coffee or alcohol, or exercise vigorously for eight hours before the test. Water is permitted before this exam.

What happens during the test?

This test requires five blood samples and often five urine specimens. Blood is drawn from a vein in your arm for an initial fasting blood sample. A urine specimen is collected immediately following. You then drink a solution containing dissolved sugar. Blood samples are drawn at 30 minutes, one hour, two hours and three hours after you drink the solution. Urine specimens are collected at the same intervals.

You may feel some discomfort from the needle punctures. If you feel faint at any time, lie down.

Does the test have risks?

If you develop severe hypoglycemia, the test is discontinued and you'll be given a drink containing orange juice with extra sugar to normalize your blood sugar. Signs and symptoms associated with hypoglycemia include nervousness, irritability, confusion, weakness, pallor, clammy skin and excessive sweating. If you have these symptoms, tell someone at the testing center. If necessary, you may receive intravenous sugar to reverse the reaction.

What are normal results?

If you don't have diabetes, your glucose levels rise and then fall quickly. Normal blood glucose levels peak at less than 160 milligrams of glucose per deciliter (mg/dL) to 180 mg/dL of blood within 30 minutes to one hour after consuming the oral sugar solution. Blood glucose returns to fasting levels or lower within two hours. Urine sugar tests remain negative throughout.

What do abnormal results mean?

Glucose levels of 200 mg/dL of higher at two hours usually indicate diabetes. Glucose levels between 140 mg/dL and 199 mg/dL at two hours indicate impaired glucose tolerance.