oral glucose tolerance test

Everything to Know about Oral Glucose Tolerance Test

You may be asked to take a version of an oral glucose tolerance test (OGTT) if you are pregnant or at risk for type 2 diabetes. How is the test conducted and what do the results mean?

What is a glucose tolerance test?

A glucose tolerance test will measure how your body’s cells absorb glucose after you consume sugar and if there are any signs of diabetes.

Typically, doctors call for a glucose tolerance test to diagnose gestational diabetes, or diabetes in someone who is pregnant. 

Gestational diabetes happens during pregnancy. It affects women who have not had diabetes prior to being pregnant but begin to experience high blood sugar levels



Prior to taking the test, it is important that you eat and drink like usual. The preparation for the oral glucose tolerance test will require an overnight fast of eight to 16 hours, but you can still carry on with your usual daily activities. However, the morning of the test, you should not consume any caffeine.


The testing is performed differently for those who are pregnant versus those who are not. There are two different tests available that your doctor may use if you are pregnant and being tested for gestational diabetes.

The first test is identical to the two-hour test used to diagnose type 1, in which you have blood drawn at the one-hour mark and again at the two-hour mark. The second test will have two different steps. The first is a one-hour screening that is followed by a three-hour test if the previous test came back with elevated results.

After your blood is drawn to test your fasting glucose, you will then drink a solution with 50 grams of sugar. After an hour, a nurse will take a blood sample. The second step follows only if the first gives a positive result.



Different test results can mean different things. Your doctor will share your results with you after the test is complete. The outcome may be one of the following:

  • Normal response – A response is considered normal when the two-hour test shows glucose levels less than 140 mg/dl.
  • Impaired glucose tolerance (IGT) – Impaired glucose tolerance happens when fasting plasma glucose is less than 126 mg/dl.
  • Diabetes – After two tests performed on separate days, if both show high blood glucose levels, a person is considered to have diabetes. 
  • Gestational diabetes – Pregnant women that have a fasting plasma glucose over 92 mg/ml or a two-hour glucose level that is more than 153 mg/dl will typically be diagnosed with gestational diabetes.

If you are pregnant, there is no need to fear or eat differently in order to “pass” the test. If you do have gestational diabetes, you will talk with your doctor about a plan to manage it. After your pregnancy, your body will go back to normal and you’ll no longer have gestational diabetes.

If you have symptoms of high blood sugar, talk to your doctor about getting an oral glucose tolerance test.