Glucose is a type of sugar found in fruits and many other foods, it is the main source of energy used by the body. Most of the sugars and starches (carbohydrates) a person eats are turned into glucose. The body can use glucose for energy or store it in the liver and muscles as glycogen.

A insulin helps the body use and control the amount of glucose in the blood. The pancreas produces insulin and releases it into the blood when the amount of glucose in the blood rises. People who do not produce enough insulin or cannot use it properly have high blood sugar levels and develop diabetes. Normally, blood glucose levels increase slightly after you eat a meal. An increase in blood glucose causes the pancreas to release insulin, which causes glucose to enter cells, thereby preventing blood glucose levels from getting too high.

The oral glucose tolerance test measures the body's ability to use glucose. After a person drinks a specific amount of glucose, its levels are measured in blood samples over a period of several hours. Normally, blood glucose levels peak within an hour and then begin to drop.

This test is done on blood samples taken from a vein.

Oral Glucose Tolerance Test
Results

Normal

Normal values may vary from lab to lab.

Greater than normal values may mean

• For a pregnant woman, gestational diabetes may be present if results of the screening test (using 50 grams of glucose) are higher than 140 mg/dL. In this case, a complete glucose tolerance diagnostic test (using 100 grams of glucose) should be done.
• For a pregnant woman, the diagnostic test may indicate gestational diabetes if two or more of the following results occur:
  • The fasting blood glucose value is greater than 95 mg/dL.
  • The 1-hour value is greater than 180 mg/dL.
  • The 2-hour value is greater than 155 mg/dL.
  • The 3-hour value is greater than 140 mg/dL.
  • For a nonpregnant person, higher than normal blood glucose levels usually indicate diabetes. The higher the level, the more likely it is that the person has diabetes. The American Diabetes Association considers diabetes to be present if:
    • The fasting glucose level is above 126 mg/dL on two occasions.
    • A random glucose level is above 200 mg/dL and signs or symptoms of diabetes are present.
    • At least two glucose tolerance diagnostic test values are above 200 mg/dL. If the results of the glucose tolerance diagnostic test are high, the test may be repeated on a different day to confirm the diagnosis. A fasting value between 109 mg/dL and 126 mg/dL or a 2-hour blood glucose level between 140 and 200 mg/dL is borderline. This result may be called “impaired glucose tolerance” and indicates that a person is at high risk for developing diabetes.
  • Other conditions that can cause high blood glucose levels: severe stress, some medications, Cushing's syndrome, acromegaly, pheochromocytoma, or hemochromatosis.

  Lower than normal values may mean

  • Glucose levels that fall below 40 mg/dL may indicate a tumor in the pancreas that produces abnormally high amounts of insulin (called an insulinoma) or a problem (such as sprue) that prevents the intestines from absorbing the nutrients from food.
  • Other causes of very low glucose levels include Addison's disease, an underactive thyroid gland or pituitary gland, or severe liver disease (cirrhosis).