A glucose tolerance test measures how well your body’s cells are able to absorb glucose, or sugar, after you ingest a given amount of sugar. Doctors use fasting blood sugar levels and hemoglobin A1c values to diagnose type 1 and type 2 diabetes, and prediabetes. A glucose tolerance test can also be used. Doctors primarily use a glucose tolerance test to diagnose gestational diabetes.
Doctors often diagnose type 1 diabetes quickly because it usually develops quickly and involves high blood sugar levels. 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 doesn’t have diabetes before pregnancy has high blood sugar levels as a result of the pregnancy. The American Diabetes Association estimates that gestational diabetes occurs in 9.2 percent of pregnancies.
Doctors should screen all 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 this test between the 24th and 28th week of your pregnancy. Your doctor may also recommend that you have this test earlier if you’re having the symptoms of prediabetes or diabetes.
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-blockers, diuretics, and antidepressants, can interfere with the results.
- Abstain from food 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.
The test can take place in your doctor’s office or 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 also called a fasting glucose test.
The test will vary depending on whether you’re being tested for diabetes or gestational diabetes.
Type 1 or 2 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 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. At the two-hour mark, they’ll ask you for another blood sample.
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, except that you’ll have a blood draw at both the one-hour and the two-hour mark. The second test involves a one-hour screening and then a three-hour glucose tolerance test if the one-hour screening levels are elevated.
After drawing a 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 is generally only conducted if the first step has a positive result. Step two is a is a three-hour version of the OGTT used in the one-step approach above. In the three-hour version of the 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.
These tests have no serious risks. If they’re testing you 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:
- stomach discomfort
For diagnosing type 2 diabetes, your doctor may ask you to test again on a different day if your test shows higher-than-normal glucose levels. You won’t retest for gestational diabetes. Doctors use standard glucose values to diagnose prediabetes, diabetes, and gestational diabetes.
Your doctor will use the following values in milligrams/deciliter (mg/dL) to diagnose diabetes in a 75-gram OGTT:
|When blood is drawn||For prediabetes||For diabetes||For gestational diabetes|
|Fasting||100-125 mg/dL||126 mg/dL or greater||greater than 92 mg/dL|
|After 1 hour||greater than 180 mg/dL|
|After 2 hours||140-199 mg/dL||200 mg/dL or greater||greater than 153 mg/dL|
Only one value needs to be elevated to be diagnostic for diabetes or gestational diabetes.
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. 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 you with gestational diabetes.
|When blood is drawn||Diagnostic levels|
|Fasting||Greater than 95 mg/dL|
|After 1 hour||Greater than 180 mg/dL|
|After 2 hours||Greater than 155 mg/dL|
|After 3 hours||Greater than 140 mg/dL|
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 you with 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 delivery, premature delivery, and other complications, such as preeclampsia. Your doctor will work with you to develop a treatment plan that works best for you.