Definition

The oral glucose tolerance test is a diagnostic procedure used to evaluate how your body processes sugar (glucose). This test is particularly useful in identifying abnormalities in glucose metabolism that can arise after eating. It is often able to detect these issues even before fasting blood sugar levels show any signs of irregularity.

When you consume food, your blood sugar levels naturally rise. In response, your pancreas releases insulin, a hormone that helps transfer glucose from the bloodstream into your body’s cells, where it can be used as energy or stored. Afterward, your blood sugar levels return to their normal state.

For those with type 2 diabetes, the body is unable to effectively use insulin. Consequently, glucose accumulates in the blood and is not utilized by the cells for energy. This leads to elevated blood sugar levels, which can cause damage to blood vessels. Over time, uncontrolled diabetes can result in complications, including heart disease, nerve damage, vision problems, and kidney damage.

Indication

The glucose tolerance test is primarily used to screen for type 2 diabetes mellitus. There is also a modified version of this test used to diagnose gestational diabetes, a type of diabetes that develops during pregnancy.

Contraindication

There are no absolute contraindications for the glucose tolerance test. However, certain conditions can make it difficult to draw blood samples, such as infections in the arm, the presence of blood vessel grafts, bruising, or hardened arteries. If any of these conditions are present, blood samples may need to be taken from other areas of the body.

Preparation Before the Test

Before undergoing the glucose tolerance test, it is important to maintain a normal diet and hydration for several days leading up to the test. If you are ill or on any regular medications, inform your doctor, as these factors can influence the test results.

For 8 hours prior to the test, you must not eat or drink anything, which is typically accomplished by fasting overnight. Scheduling the test for the morning after fasting is the most effective approach. Your doctor may provide additional instructions if there are any specific preparations required for your case.

Test Procedure

The glucose tolerance test is conducted in several stages. Upon arrival at the testing facility, a health professional will first take a blood sample from a vein, usually in the arm, to measure your fasting blood glucose levels. This sample will serve as a baseline to assess your body’s response to glucose intake during the rest of the test.

If you are undergoing a Type 2 Diabetes Mellitus test:

• You will be asked to drink approximately 237 mL of a glucose syrup solution that contains 75 grams of sugar.
• After two hours, your blood glucose levels will be tested again.

For gestational diabetes, the American College of Obstetricians and Gynecologists (ACOG) recommends a glucose challenge test as a screening method for low-risk pregnant women between 24-28 weeks of pregnancy. However, if you are at higher risk of developing gestational diabetes, the screening can be performed earlier.

If suspicious results are observed during this screening, pregnant women may be advised to take a 3-hour oral glucose tolerance test. This procedure follows a similar process as the standard test, but your blood glucose levels will be re-measured at 1, 2, and 3 hours after consuming the sugar solution.

Once the test is completed, you can resume your normal daily activities.

Normal and Abnormal Values

The results of the oral glucose tolerance test are typically expressed in milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L).

For Type 1 and Type 2 diabetes mellitus, the test results are interpreted as follows:

• Normal results: <140 mg/dL

• Impaired results: 140 - 199 mg/dL

• Abnormal results: ≥200 mg/dL

For gestational diabetes, the normal ranges for the different time intervals are:

• One-hour glucose: <180 mg/dL (10 mmol/L)

• Two-hour glucose: <155 mg/dL (8.6 mmol/L)

• Three-hour glucose: <140 mg/dL (7.8 mmol/L)

Results and Recommendations (Follow-up Tests)

If your blood glucose levels are within normal ranges, with a result under 140 mg/dL (7.8 mmol/L), your glucose tolerance is considered normal. If the test shows a result between 140-199 mg/dL (7.8 - 11 mmol/L), it indicates that you have impaired glucose tolerance, also known as prediabetes. Prediabetes is a condition where your blood glucose levels are higher than normal but not yet high enough to be classified as diabetes. Individuals with prediabetes are at a greater risk of developing Type 2 diabetes mellitus, as well as heart disease, even if they do not have diabetes.

If your test results exceed 200 mg/dL (11.1 mmol/L), this suggests that you have diabetes. If the oral glucose tolerance test results indicate that you have Type 2 diabetes, your doctor may recommend repeating the test on a different day or may suggest additional blood tests to confirm the diagnosis. Factors such as illness, physical activity, and certain medications can all impact the accuracy of the test.

For pregnant women undergoing a glucose tolerance test, if the results of one blood glucose test exceed normal levels, your doctor will likely ask you to repeat the test in about 4 weeks. If two or more of your results are above normal, you may be diagnosed with gestational diabetes.

Consult the Right Doctor

If your oral glucose tolerance test results are abnormal, it’s important to consult with a general practitioner who will interpret the results and provide appropriate next steps.

For pregnant women with abnormal results from the oral GTT, it’s recommended to consult with your obstetrician for further evaluation and management.

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