Glucose Tolerance Test
- Also Known As:
- Oral Glucose Tolerance Test
Test Quick Guide
The glucose tolerance test is a method to judge how well your body is able to metabolize glucose.
Glucose is the type of sugar your body uses to supply energy to its cells. Your body breaks down the food you eat into glucose that then enters your bloodstream. A hormone called insulin, made by your pancreas, helps bring the glucose from your bloodstream into the cells of your body.
If your body’s cells have difficulty accepting insulin, a condition called insulin resistance, or if your body cannot produce enough insulin, the glucose levels in your blood will be too high. High blood glucose can be a sign of prediabetes or diabetes. Diabetes is a serious medical issue that can damage the organs and tissues of the body if it is not controlled.
By measuring how your body processes glucose, the glucose tolerance test can be used with other tests to detect or confirm prediabetes and diabetes.
About the Test
Purpose of the test
The glucose tolerance test is one way to assess how well your body metabolizes glucose. Your doctor may have several different reasons for recommending this test:
- Diagnosing prediabetes and diabetes: Diagnosis is the process of determining what is causing changes in your health that you or your doctor may have observed. The glucose tolerance test may be ordered if you are showing signs or symptoms of diabetes, especially if other tests for diabetes are inconclusive.
- Screening for prediabetes and diabetes: Screening is checking for a disease before it causes signs and symptoms. Your doctor may want you to be screened for diabetes if you are over 45 years old or are at a greater-than-average risk of the disease. In general, other types of diabetes tests are more commonly used for screening than the glucose tolerance test. However, the glucose tolerance test is frequently used to screen for diabetes in pregnant women.
- Screening for and diagnosing gestational diabetes: Gestational diabetes is a form of diabetes that can develop in women while they are pregnant. If you are pregnant, your doctor may suggest this testing, which is described in our guide to Glucose Tests for Gestational Diabetes.
What does the test measure?
The glucose tolerance test measures the amount of glucose that remains in your bloodstream after fasting and then after drinking a sugary drink at fixed intervals. Blood glucose levels are typically measured in milligrams per deciliter, or mg/dL.
Finding a Glucose Tolerance Test
How to get tested
Your doctor is typically the one to order a glucose tolerance test. The test may be conducted in your doctor’s office or in a hospital, clinic, or laboratory.
Can I take the test at home?
There is no at-home glucose tolerance test at this time. This test requires several blood draws and laboratory analysis of blood samples, so it needs to be done in the setting of a clinic or laboratory.
How much does the test cost?
A glucose tolerance test may cost different amounts depending on the type of health insurance you have and where the test is conducted. The full cost of the test may take into account the price of the office visit, the technician’s charges for obtaining your blood samples, and the fees for the laboratory to perform analysis on your samples.
When your doctor orders a glucose tolerance test for you, your insurance provider will usually cover the costs and fees associated with the test. You may need to cover some expenses out of pocket, such as deductibles and copayments. You can get more information about the cost of your glucose tolerance test by talking to your doctor, your health insurance company, or the staff at the office or facility where the test will be performed.
Taking a Glucose Tolerance Test
When you receive a glucose tolerance test, a phlebotomist will draw a sample of your blood through a needle from a vein in your arm after you have fasted for at least eight hours, typically overnight. Then you will drink a sweet drink, and the technician will take more blood samples over the next two to three hours.
Before the test
You will be required to fast for at least eight hours before you take the test. This means you will need to go without any food or any drink besides water. Since the test is often scheduled for the morning, most of your fasting can happen while you sleep the night before.
You can generally follow a normal diet up until the time you begin fasting. Your doctor may ask you to make sure you consume at least 150 grams of carbohydrates each day in the three days before your glucose tolerance test.
If you take any prescription medications or over-the-counter drugs, be sure to discuss these with your doctor before you take the test. Your doctor can let you know whether any of these might alter the results of the test. They can also advise you whether you will need to stop taking any medications during the time you are fasting.
To ensure the most reliable results, ask your doctor for any specific pretest preparations and carefully follow those instructions on the days leading up to your glucose tolerance test.
During the test
Glucose tolerance testing usually takes place in the morning. You may need to stay at the testing site for up to three hours.
The phlebotomist who gives you the test will start by drawing a small amount of your blood from a vein, most often in your arm. The technician may tie a band around your upper arm so the vein is easier to see. After cleaning the skin over your vein with a sterile wipe, a small needle will be inserted through the skin and into the vein. The sample of blood goes into a tube attached to the needle.
When the needle comes out, the phlebotomist will release the band on your arm and cover the injection site with a small bandage. Some people experience minor pain or discomfort when the needle goes in or comes out, but this usually does not last very long.
Once the first blood sample is taken, you will drink a bottle of liquid containing a defined amount of glucose. In adults, the drink usually contains 75 grams of glucose. In children, the amount is calculated based on body weight with a maximum of 75 grams. This drink tastes sweet like a sugary soda.
After you drink the glucose solution, your blood will be drawn several more times at regular intervals over the next two or three hours. You should not eat anything during this time, and you typically will remain in the waiting area.
After the test
After the series of blood draws is complete, you may experience slight pain where the blood was taken. You can generally resume normal activities after your glucose tolerance test is performed. When the test is completed, you will be able to eat again. You may wish to take a snack with you to the testing site to have on hand after your last blood draw is finished.
Glucose Tolerance Test Results
Receiving test results
You will usually get the results of your glucose tolerance test a few business days after you take the test. You may be able to access your test results online, or you may receive results by email or through postal mail. Your doctor may reach out by phone or email to discuss the test results or may schedule an appointment to go over the results in person.
Interpreting test results
Laboratory technicians will analyze the glucose level in each blood sample. Values for each of the separate timed samples will be included in your lab report.
The laboratory analysis will help determine if your glucose level in any of the blood samples is abnormally high. In particular, your blood glucose level measured two hours after you drink the glucose solution can be used in diagnosing diabetes or prediabetes.
The National Institute of Diabetes and Digestive and Kidney Disorders and the American Diabetes Association, among other organizations, list the following reference ranges for this two-hour value:
- Normal: Under 140 mg/dL
- Prediabetes: 140 to 199 mg/dL
- Diabetes: At or over 200 mg/dL
While this test can be used to diagnose both type 1 and type 2 diabetes, one test alone does not usually determine if you have diabetes. The doctor normally considers whether you have symptoms of diabetes, the results of other blood tests, or a repeated glucose tolerance test to arrive at a definitive diagnosis.
As a result, the best way to understand what your glucose tolerance test results mean is to discuss them with your doctor. Your doctor can explain how your results compare to the reference ranges, whether you will need a repeat test or other follow-up tests, and what further steps you may need to take for your health.
People diagnosed with prediabetes have a greater risk of developing diabetes and other health conditions over the years following their diagnosis. If your test results indicate prediabetes, your doctor may recommend dietary and lifestyle changes that can lower the chance that your condition will progress to diabetes or that can delay this progression.
If you are diagnosed with diabetes, your doctor may go over the steps you need to take to manage your condition. These steps can include monitoring your blood glucose at home and changing your diet and exercise routine. Your doctor may also prescribe medication and regular follow-up visits.
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