What is a glucose tolerance test?
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
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.
needs a glucose tolerance test?
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.
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-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
of a glucose tolerance test
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
Some people find the glucose drinks difficult to tolerate, especially those
with higher levels of sugar. You may experience:
- stomach discomfort
of glucose tolerance test
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:
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.
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
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
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.