Glucose Tolerance Test: Purpose, Procedure, and Risks

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What is a glucose tolerance test?

A glucose tolerance test measures how well your body’s cells are able to absorb glucose (sugar) after you consume a specific amount of sugar.
Doctors use fasting blood sugar levels and hemoglobin A1c values to diagnose type 1 and type 2 diabetes as well as prediabetes.
Doctors primarily use a glucose tolerance test to diagnose diabetes during pregnancy (called gestational diabetes).
Doctors often diagnose type 1 diabetes quickly because it usually develops rapidly and involves high blood sugar levels and symptoms.
Type 2 diabetes, on the other hand, often develops over years. Type 2 diabetes is the most common form of diabetes, and it usually develops during adulthood.
Gestational diabetes occurs when a pregnant woman who hasn’t had diabetes before pregnancy has high blood sugar levels as a result of the pregnancy. The American Diabetes Association estimates that gestational diabetes occurs in up to 9.2 percent of pregnancies in the United States.

Who needs a glucose tolerance test?

Doctors should screen all pregnant women for gestational diabetes. Gestational diabetes can cause pregnancy complications, so early detection and prompt treatment are important.
If you’re pregnant, your doctor will usually recommend that you have this test between weeks 24 and 28 of your pregnancy.
Your doctor may also recommend that you do this test earlier if you’re experiencing diabetes symptoms or if you were at risk of having diabetes before you were pregnant.

Preparing for a glucose tolerance test

Preparing for the glucose tolerance test involves the following:
  • Continue to eat a normal diet in the days leading up to the test.
  • Consult with your doctor about any medications you’re currently taking. Some medications, such as corticosteroids, beta-blockersdiuretics, and antidepressants, can interfere with the results.
  • You’ll need to fast for at least eight hours before the scheduled test. You may drink water, but avoid other beverages, including coffee and caffeinated tea, as these can interfere with the results.
  • Avoid going to the bathroom just before the procedure because you may need to provide a urine sample.
  • Bring something to read or an activity to keep you busy while you wait.

During the test

The test can take place in your doctor’s office or at a local lab. When you arrive, a technician will take a blood sample to measure your baseline glucose level. This part of the test is a type of blood glucose test called a fasting glucose test.
Next, the test will vary depending on whether you’re being tested for diabetes or gestational diabetes.

Oral glucose tolerance test for diabetes

A two-hour, 75-gram oral glucose tolerance test (OGTT) is used to test for diabetes. A healthcare provider will take a fasting lab draw of blood to test your fasting glucose level first. They’ll then ask you to drink 8 ounces of a syrupy glucose solution that contains 75 grams of sugar.
You’ll then wait in the office for two hours. The healthcare provider will draw blood at the one- and two-hour marks.

Gestational diabetes testing

Your doctor may use two tests to help them determine if you have gestational diabetes. The first test uses the same two-hour test already described, and you’ll have a blood draw at both the one- and the two-hour mark.
The second test may involve two steps: first, a one-hour screening and then, a three-hour glucose tolerance test if the one-hour screening levels are elevated.
After a blood draw to test fasting glucose, you’ll drink a solution with 50 grams of sugar. An hour later, you’ll give a blood sample. A lab technician will use this sample to measure your blood sugar level.
The second step of this test is generally only conducted if the first has a positive result. It involves a three-hour version of the OGTT.
In the three-hour test, a healthcare provider will ask you to consume a syrupy glucose solution that contains 100 grams of sugar. They’ll draw your blood when you’re fasting and at the one-, two-, and three-hour marks after you’ve drunk the glucose solution.
By taking several samples of your blood as your body processes the sugary drink, your doctor will be able to tell how well your body can handle a sugar challenge.

Risks of a glucose tolerance test

These tests have no serious risks. If you’re being tested for gestational diabetes, this test has no associated serious risks for your or your baby.
Breaking the skin barrier can slightly increase your risk of infection. Watch for signs of infection, such as redness and swelling around the puncture site, and fever. You may also feel faint or dizzy from not eating. It’s a good idea to eat after the test.
Some people find the glucose drinks difficult to tolerate, especially those with higher levels of sugar. You may experience:
  • nausea
  • stomach discomfort
  • diarrhea
  • constipation

Results of glucose tolerance test

For diagnosing type 2 diabetes, your doctor may ask you to test again on a different day if your results show higher-than-normal glucose levels. You won’t retest for gestational diabetes.
Your doctor will use the following values in milligrams per deciliter (mg/dL) to diagnose gestational diabetes in a 75-gram OGTT. Only one value needs to be elevated to diagnose diabetes or gestational diabetes.
When blood is drawnFor prediabetesFor diabetesFor gestational diabetes
Fasting100–125 mg/dL126 mg/dL or greatergreater than 92 mg/dL
After 1 hourgreater than 180 mg/dL
After 2 hours140–199 mg/dL200 mg/dL or greatergreater than 153 mg/dL

Gestational diabetes: Two-step approach

If your one-hour results are equal to or greater than 135 or 140 mg/dL, your doctor will ask you to proceed to the second step of the test. As mentioned above, the second step involves ingesting 100 grams of sugar.

If two of your four blood draw levels are higher than those listed below, your doctor will diagnose gestational diabetes.
When blood is drawnDiagnostic levels
Fastinggreater than 95 mg/dL
After 1 hourgreater than 180 mg/dL
After 2 hoursgreater than 155 mg/dL
After 3 hoursgreater than 140 mg/dL

After the glucose tolerance test

For diabetes, your doctor may recommend that you take more tests before they make a diagnosis. No other testing will be done to diagnose gestational diabetes.
If your doctor diagnoses prediabetes or diabetes, they’ll recommend that you make diet and exercise changes. They may also prescribe diabetes medications as needed.
Doctors treat gestational diabetes with diet and activity, and your doctor will add medication to your treatment if you need it. Your doctor will ask you to monitor your blood sugar levels every day to make sure they’re within the recommended targets.
If you have gestational diabetes, you should start treatment right away.
Unmanaged diabetes can lead to having a larger-sized baby, which may cause complications during deliverypremature delivery, and other complications, like preeclampsia. Your doctor will work with you to develop a treatment plan that tailored to your needs.

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