Glucose Tolerance Test
Blood samples drawn from a vein at timed intervals
A glucose tolerance test requires that you fast for 8 to 12 hours (overnight) before the first blood sample is drawn. You will then be given a liquid containing 75 grams of glucose to drink (or 1.75 gram of glucose per kilogram of body weight if you are a child, up to a maximum of 75 grams). Subsequent blood samples are drawn 2 hours after you begin to drink the glucose drink. You have 5 minutes to drink the glucose drink.
Glucose is the primary energy source for the body's cells and the only short-term energy source for the brain and nervous system. A steady supply must be available for use, and a relatively constant level of glucose must be maintained in the blood. A glucose tolerance test measures the level of glucose in your blood when you are fasting and then 2 hours after drinking a liquid containing a specific amount of glucose.
During digestion, the carbohydrates that you eat are broken down into glucose (and other nutrients). They are absorbed by the digestive tract, move into the blood, and circulate throughout the body. Normally, blood glucose rises slightly after a meal and the hormone insulin is released by the pancreas into the blood in response. The amount of insulin released corresponds to the size and content of the meal. Insulin helps transport glucose into the body's cells, where it is used for energy. As glucose moves into the cells and is broken down (metabolized), the blood glucose level drops and the pancreas responds by decreasing the release of insulin. (The glucose tolerance test does not require the body to break down carbohydrates into glucose since the glucose in the liquid that you drink is already free.)
If this glucose/insulin blood feedback system is working properly, the amount of glucose in the blood remains fairly stable. If the feedback system is disrupted and the glucose level in the blood rises, then the body tries to restore the balance by increasing insulin production If the body's pancreatic beta cells can do so.
Diabetes is the most common disease resulting from an imbalance between glucose and insulin.
- Type 1 diabetes results when the body is not able to produce sufficient insulin to control blood glucose levels. Usually in type 1 diabetes, the cells that produce insulin (beta cells) have been destroyed by the person's own immune system.
- Type 2 diabetes results from a combination of insulin resistance (the body does not react normally to insulin) and a relative decline in insulin production.
- Some women may develop gestational diabetes, which is high blood glucose that occurs only during the second or third trimesters of pregnancy. (For more information, see the article on Glucose Testing for Gestational Diabetes.)
How is the test used?
A glucose tolerance test (GTT) may be used to help diagnose prediabetes and diabetes. In some cases, a GTT may be used as a screening test, but most commonly, a fasting blood glucose (FBG) or hemoglobin A1c test is used for diabetes screening.
One common protocol is the 2-hour glucose tolerance test (GTT). For this test, you may have a fasting glucose test done, then you drink a 75-gram glucose drink (or 1.75 grams per kilogram of body weight if you are a child). Another blood sample is drawn 2 hours after you begin to drink the glucose drink. This protocol "challenges" your body to produce insulin to prevent hyperglycemia while returning the glucose level back to normal.
The GTT is more complicated to perform than other tests used to detect diabetes, such as a FBG or hemoglobin A1c, but is more sensitive and can detect diabetes earlier. Thus, the GTT may sometimes be used to screen for diabetes in certain patient populations (e.g., candidates for kidney transplants).
If the test is used for screening and the initial glucose screening result is abnormal (e.g., hyperglycemia is present at some time during the test), the test may be repeated. The second test result must also be abnormal (e.g., hyperglycemia is present at some time during the test) to confirm a diagnosis of diabetes. Alternatively, an abnormal screening result may be followed by more definitive testing. During the GTT, if only the fasting glucose is in the hyperglycemic range (which is unusual), the repeat test need only be a fasting glucose test.
When is it ordered?
What does the test result mean?
Normally, the 75-gram glucose drink raises your blood glucose level, which stimulates the pancreas to release insulin into the blood. Insulin allows the glucose to be taken up by cells. As time passes, the blood glucose level is expected to decrease again back to normal. When you are unable to produce enough insulin, or if the body's cells are resistant to its effects (insulin resistance), then less glucose is transported from the blood into cells and the blood glucose level remains high.
The following table summarizes the meaning of GTT results.
2-Hour Oral Glucose Tolerance Test (OGTT)—non pregnancy
Levels applicable except during pregnancy. Samples drawn fasting and 2 hours after a 75-gram glucose drink. Glucose Level Interpretation Fasting Less than 100 mg/dL (5.5 mmol/L) Normal glucose tolerance 100 to 125 mg/dL (5.6 to 6.9 mmol/L) Impaired fasting glucose (prediabetes) 126 mg/dL (7.0 mmol/L) and above Elevated (hyperglycemia is present) 2 hour Less than 140 mg/dL (7.8 mmol/L) Normal glucose tolerance From 140 to 199 mg/dL (7.8 to 11.1 mmol/L) Impaired glucose tolerance (prediabetes) Equal to or greater than 200 mg/dL (11.1 mmol/L) Elevated (hyperglycemia is present)
Note: The most severe elevation in glucose is used to characterize your status: normal (e.g., nondiabetic), prediabetes (impaired fasting glucose and/or impaired glucose tolerance) is present, or diabetes (hyperglycemia) is present. If you have symptoms of diabetes, hyperglycemia that is either fasting or at 2 hours will establish the diagnosis of diabetes. If you are asymptomatic, diabetes is diagnosed when any 2 abnormal results are present on the same day or 2 abnormal results are present on 2 different days.
What does the glucose drink taste like?
Can I drink water before or during the test?
What other lab tests may be done?
Hemoglobin A1c is an indicator of long-term glucose control that is commonly ordered in patients with prediabetes and diabetes and can also be used as a screening test for diabetes. A fasting blood glucose is another test that may be used for diabetes screening and diagnosis. Other less common tests, such as islet autoantibodies, insulin, and C-peptide, may sometimes be performed to help determine the cause of abnormal glucose levels, to distinguish between type 1 and type 2 diabetes, and to evaluate insulin production. Islet autoantibodies, insulin, and C-peptide are not measured to diagnose diabetes.
How is diabetes treated?
For type 2 diabetes, which is the most common type of diabetes, losing excess weight, eating a healthy diet that is high in fiber and restricted in carbohydrates, and getting regular amounts of exercise may be enough to lower your blood glucose levels. In many cases, however, medications may be necessary to achieve the desired glucose level. With type 1 diabetes (and with type 2 diabetes that does not respond well enough to oral medications), insulin injections several times a day are necessary. Insulin can also be given by a pump continuously subcutaneously. See the article on Diabetes for more on treatment.
Is there anything else I should know?
Some medications can increase or decrease your blood glucose level. Be sure to tell your healthcare practitioner about any prescribed or over-the-counter drugs or supplements you take.