What is Glucose Tolerance Test? Purpose, Levels/Procedure & Results

A glucose tolerance test measures how well your body processes sugar over time. It's a simple blood test that helps doctors understand if your body can handle glucose properly, which is essential for diagnosing conditions like diabetes and prediabetes.

Think of it as a stress test for your body's sugar-handling system. During the test, you'll drink a sweet solution, and then your blood will be checked at specific intervals to see how your glucose levels rise and fall. This gives your healthcare provider a clear picture of your metabolic health.

What is glucose tolerance test?

A glucose tolerance test (GTT) is a medical test that measures your body's ability to process glucose, which is the main type of sugar in your blood. The test shows how your blood sugar levels change over time after you consume a specific amount of glucose.

There are two main types of glucose tolerance tests. The oral glucose tolerance test (OGTT) is the most common, where you drink a glucose solution and have your blood tested multiple times. The intravenous glucose tolerance test (IVGTT) involves glucose being injected directly into your vein, but this is rarely used today.

During a standard OGTT, you'll typically have blood drawn before drinking the glucose solution (fasting level), then again at one hour, two hours, and sometimes three hours afterward. This pattern helps doctors see exactly how your body responds to sugar intake.

Why is glucose tolerance test done?

Doctors order glucose tolerance tests primarily to diagnose diabetes and prediabetes when other tests aren't conclusive. This test is especially helpful when your fasting blood sugar levels are borderline or when you have symptoms that suggest blood sugar problems.

Pregnant women often receive a glucose tolerance test between 24 and 28 weeks to check for gestational diabetes. This condition can develop during pregnancy and affects how your body processes sugar, potentially impacting both you and your baby's health.

Your doctor might also recommend this test if you have risk factors for diabetes. These include being overweight, having a family history of diabetes, being over 45 years old, or having high blood pressure. The test can catch problems early, even before obvious symptoms appear.

Sometimes, the test helps monitor how well diabetes treatments are working. If you've already been diagnosed with diabetes or prediabetes, your healthcare provider might use periodic glucose tolerance tests to track your progress and adjust your treatment plan.

What is the procedure for glucose tolerance test?

The glucose tolerance test procedure is straightforward but requires some time and preparation. You'll start by having a small amount of blood drawn from your arm to measure your fasting glucose level, which serves as your baseline.

Next, you'll drink a glucose solution that tastes quite sweet, similar to a very sugary soft drink. The standard solution contains 75 grams of glucose for adults, though pregnant women may receive a different amount. You'll need to finish the entire drink within five minutes.

After drinking the solution, you'll wait in the testing area while your body processes the glucose. Here's what happens during the waiting period:

• You'll have blood drawn again at one hour after drinking the solution
• Another blood sample will be taken at the two-hour mark
• Some tests may require a third blood draw at three hours
• You'll need to stay in the testing facility during the entire process
• You can't eat, drink, or smoke during the test period

Each blood draw takes just a few minutes, and the entire test usually lasts about three hours. Most people find the waiting time is the most challenging part, so consider bringing a book or something quiet to keep you occupied.

How to prepare for your glucose tolerance test?

Proper preparation is crucial for accurate glucose tolerance test results. You'll need to fast for at least 8 to 12 hours before the test, which means no food, drinks (except water), or anything with calories during this time.

Your diet in the days leading up to the test can affect your results. For the three days before your test, eat normally and don't try to restrict carbohydrates or change your eating habits. Your body needs to be in its usual state for the test to be meaningful.

Here are the key preparation steps to follow:

• Continue taking your regular medications unless your doctor specifically tells you to stop
• Avoid intense exercise or physical stress on the day of the test
• Get a good night's sleep before the test day
• Bring a list of all medications and supplements you're taking
• Wear comfortable clothing with loose sleeves for easy blood draws
• Plan to stay at the testing facility for the entire test duration

Let your healthcare provider know about any medications you're taking, as some can affect blood sugar levels. They'll advise you whether to continue or temporarily stop any medications before the test.

How to read your glucose tolerance test?

Understanding your glucose tolerance test results involves looking at your blood sugar levels at different time points. Normal results show that your blood sugar rises after drinking the glucose solution but returns to healthy levels within two hours.

For a standard oral glucose tolerance test, here are the typical result ranges:

• Fasting (before drinking glucose): Normal is less than 100 mg/dL
• One hour after glucose: Normal is less than 180 mg/dL
• Two hours after glucose: Normal is less than 140 mg/dL
• Three hours after glucose: Normal is less than 140 mg/dL

Prediabetes is diagnosed when your two-hour result falls between 140 and 199 mg/dL. This means your body is having some trouble processing glucose, but you don't yet have diabetes. It's a warning sign that gives you time to make lifestyle changes.

Diabetes is diagnosed when your two-hour result is 200 mg/dL or higher, or if your fasting level is 126 mg/dL or higher. These numbers indicate that your body isn't processing glucose effectively, and you'll need ongoing medical care.

For pregnant women, the thresholds are slightly different. Gestational diabetes is diagnosed if any of these values are exceeded: fasting level of 92 mg/dL, one-hour level of 180 mg/dL, or two-hour level of 153 mg/dL.

How to fix your glucose tolerance test levels?

If your glucose tolerance test results are abnormal, the good news is that you can often improve them through lifestyle changes and, when necessary, medical treatment. The approach depends on whether you have prediabetes or diabetes.

For prediabetes, lifestyle modifications can often prevent or delay the development of type 2 diabetes. Weight loss of just 5 to 7 percent of your body weight can make a significant difference. This might mean losing 10 to 15 pounds if you weigh 200 pounds.

Here are the most effective ways to improve your glucose tolerance:

• Increase physical activity to at least 150 minutes of moderate exercise per week
• Choose whole grains, lean proteins, and plenty of vegetables in your diet
• Limit refined sugars and processed foods
• Maintain a healthy weight through portion control
• Get adequate sleep, aiming for 7 to 9 hours nightly
• Manage stress through relaxation techniques or counseling

If you have diabetes, you'll likely need medication along with lifestyle changes. Your doctor might prescribe metformin or other diabetes medications to help your body use glucose more effectively. Regular monitoring and follow-up appointments will be essential.

Working with a registered dietitian can be incredibly helpful for both prediabetes and diabetes. They can create a personalized meal plan that helps stabilize your blood sugar while still being enjoyable and sustainable.

What is the best glucose tolerance test level?

The best glucose tolerance test levels are those that fall within the normal range, indicating that your body processes glucose efficiently. Optimal results show your blood sugar rising moderately after the glucose drink and returning to baseline levels within two hours.

Your ideal fasting glucose level should be between 70 and 99 mg/dL. This range shows that your body maintains stable blood sugar levels when you haven't eaten for several hours. Levels in this range suggest good metabolic health and proper insulin function.

After drinking the glucose solution, your blood sugar should peak around one hour and then gradually decline. The two-hour level should be below 140 mg/dL, with many healthcare providers preferring to see levels below 120 mg/dL for optimal health.

However, what's "best" can vary slightly based on your individual circumstances. Age, pregnancy, and certain medical conditions might influence what targets your doctor considers ideal for you. Your healthcare provider will interpret your results in the context of your overall health picture.

What are the risk factors for abnormal glucose tolerance test?

Several factors can increase your risk of having abnormal glucose tolerance test results. Understanding these risk factors can help you and your doctor determine if testing is appropriate and what results to expect.

Age is one of the most significant risk factors, with diabetes risk increasing substantially after age 45. As you get older, your body's ability to process glucose may naturally decline, making abnormal results more likely.

Here are the key risk factors that can affect your glucose tolerance:

• Being overweight or obese, especially with excess belly fat
• Having a family history of diabetes in parents or siblings
• Leading a sedentary lifestyle with little physical activity
• Having high blood pressure (140/90 mmHg or higher)
• Having abnormal cholesterol levels, particularly low HDL or high triglycerides
• Having polycystic ovary syndrome (PCOS)
• Having a history of gestational diabetes or giving birth to a baby weighing more than 9 pounds

Certain ethnic groups also have higher risks, including African Americans, Hispanic Americans, Native Americans, Asian Americans, and Pacific Islanders. This increased risk appears to be related to genetic factors combined with lifestyle and environmental influences.

Some medications can also affect glucose tolerance, including corticosteroids, certain blood pressure medications, and some psychiatric medications. If you're taking any of these, discuss with your doctor how they might influence your test results.

Is it better to have high or low glucose tolerance test results?

Lower glucose tolerance test results are generally better, as they indicate that your body is processing glucose efficiently. However, the goal isn't to have the lowest possible numbers, but rather to have results that fall within the normal, healthy range.

Normal glucose tolerance shows that your pancreas produces adequate insulin and your cells respond to it properly. This means your body can effectively move glucose from your bloodstream into your cells where it's needed for energy.

High glucose tolerance test results indicate that your body is struggling to process glucose effectively. This could mean your pancreas isn't producing enough insulin, your cells aren't responding to insulin properly, or both. These elevated results increase your risk of developing diabetes and its complications.

Very low glucose results during the test are uncommon but can sometimes occur. If your blood sugar drops significantly during the test, it might suggest reactive hypoglycemia, where your blood sugar falls too low after eating. This condition requires different management than diabetes.

What are the possible complications of low glucose tolerance test results?

Low glucose tolerance test results are generally not associated with serious complications, as they typically indicate good glucose metabolism. However, unusually low results might suggest reactive hypoglycemia, which can cause its own set of symptoms.

Reactive hypoglycemia occurs when your blood sugar drops too low within a few hours after eating. This can happen if your body produces too much insulin in response to glucose, causing your blood sugar to fall below normal levels.

Here are the potential symptoms and complications of reactive hypoglycemia:

• Shakiness, nervousness, or anxiety
• Sweating and clamminess
• Rapid heartbeat or palpitations
• Hunger and nausea
• Dizziness or lightheadedness
• Difficulty concentrating or confusion
• Irritability or mood changes

These symptoms usually resolve quickly once you eat something containing carbohydrates. However, frequent episodes can interfere with daily activities and may indicate an underlying condition that needs medical attention.

In rare cases, very low glucose levels during the test might suggest other medical conditions, such as insulinomas (insulin-producing tumors) or certain hormonal disorders. These conditions require specialized medical evaluation and treatment.

What are the possible complications of high glucose tolerance test results?

High glucose tolerance test results indicate prediabetes or diabetes, both of which can lead to serious long-term complications if not properly managed. The higher your glucose levels, the greater your risk of developing these complications over time.

Diabetes complications develop gradually and can affect multiple organ systems in your body. The good news is that maintaining good blood sugar control can prevent or delay most of these complications, which is why early detection through glucose tolerance testing is so important.

Here are the potential long-term complications of uncontrolled high blood glucose:

• Heart disease and stroke due to damaged blood vessels
• Kidney disease that can progress to kidney failure
• Eye problems, including diabetic retinopathy that can cause blindness
• Nerve damage (neuropathy) causing pain, tingling, or numbness
• Poor wound healing and increased infection risk
• Dental problems and gum disease
• Skin conditions and slower healing

The risk of these complications increases with both the level of blood glucose elevation and the duration of poor glucose control. This is why it's crucial to take abnormal glucose tolerance test results seriously and work with your healthcare provider to develop an effective management plan.

Even with prediabetes, you have an increased risk of heart disease and stroke. However, lifestyle changes at this stage can often prevent progression to type 2 diabetes and reduce your risk of complications.

When should I see a doctor for glucose tolerance test results?

You should see a doctor to discuss your glucose tolerance test results regardless of the outcome. Your healthcare provider needs to interpret the results in the context of your overall health, symptoms, and risk factors.

If your results are normal, you may not need immediate follow-up testing, but your doctor might recommend repeat testing in one to three years depending on your risk factors. Regular monitoring is important because glucose tolerance can change over time.

You should schedule an appointment promptly if your results show prediabetes or diabetes. Here's when to seek medical attention:

• Any abnormal glucose tolerance test results need professional interpretation
• If you experience symptoms like excessive thirst, frequent urination, or unexplained weight loss
• If you have recurring episodes of low blood sugar symptoms
• If you're pregnant and have abnormal results
• If you have questions about your results or what they mean for your health

Don't delay seeking medical care if you have diabetes-related symptoms, even if your test results haven't come back yet. Symptoms like extreme thirst, frequent urination, blurred vision, or slow-healing wounds need prompt medical attention.

Your doctor will help you understand what your results mean and develop a plan for managing your health going forward. This might include lifestyle counseling, medication, or referrals to specialists like endocrinologists or diabetes educators.