Do I Need an Oral Glucose Tolerance Test?

Medically Reviewed by Poonam Sachdev on April 16, 2024
Written by Shawna Seed
6 min read

An oral glucose tolerance test checks how well your body processes sugar. If the test finds your level is too high, that's a sign your cells aren't absorbing sugar the way they should be. 

Your blood sugar level can give your doctor important clues about your health.

The test can tell your doctor whether you're at risk of having diabetes or if you already have it. A shorter version of an OGTT checks for diabetes during pregnancy, which is called gestational diabetes.

Normally when you eat, your blood sugar rises. Your pancreas, a long gland deep in the belly, releases a hormone called insulin. It helps move sugar from your blood into your cells for energy and storage. Then your blood sugar goes back down to normal.

If you have type 2 diabetes, your body uses insulin poorly. Glucose builds up in your blood. This excess sugar can damage blood vessels around your body. Diabetes can lead to heart disease, nerve damage, eye disease, and kidney damage.

You might need an oral glucose tolerance test if you:

 

If you're pregnant, you'll probably have a shorter version of the oral glucose tolerance test between your 24th and 28th weeks of pregnancy. The shorter version, which takes only an hour, is called a glucose challenge test.

Your doctor might want you to take the test earlier in your pregnancy if your risk of gestational diabetes is elevated. Risk factors include: 

  • Having gestational diabetes in an previous pregnancy
  • A family history of diabetes
  • Obesity
  • Another medical condition that raises your risk of diabetes, such as PCOS or metabolic syndrome

If your doctor is concerned about your risk, or if you show an elevated level on the glucose challenge test, your doctor probably will have you take another version of the test.

In the days leading up to the test, eat the way you normally do. 

You shouldn't eat anything for 8 hours before the test. You may want to schedule it first thing in the morning, so that you can fast overnight. You shouldn't drink anything except water during that 8 hours. When you schedule the test, talk to your doctor about any medications you take.

 

 

You'll get the OGTT at your doctor's office, a clinic, hospital, or lab. 

The first step is taking a blood sample. This will establish what your fasting blood glucose level is. 

Then you'll drink a large glass of a sugary concoction. If the test is being done on a child, the amount they drink is adjusted to reflect their body weight. 

If the test is checking for type 2 diabetes, you'll have blood drawn again 2 hours after you drink the liquid. 

If you're pregnant, you might have to do a longer version of the oral glucose tolerance test. Your doctor might order this because you're at high risk of getting gestational diabetes or because your reading on the 1-hour glucose challenge test caused concern. 

You'll also fast for 8 hours and have a blood sample taken to establish your fasting blood glucose level. Then you'll have blood samples taken 1, 2 and 3 hours after you drink the sugary solution. 

You'll probably have to wait at the doctor's office or lab for the entire test period. 

Once the test is done, you can go back to your normal routine right away. 

 

 

The OGTT has very few issues. Some people have minor side effects from the sugary drink or from the needle stick.

Side effects from the drink include:

Possible problems from the blood test include:

  • Excess bleeding
  • Fainting
  • Infection
  • More than one attempt to find a vein, which can hurt a little
  • Bruising or soreness at the needle site

Your blood glucose level should rise after you finish the sugary drink. Then it should go back to normal, as insulin moves glucose into your cells. If your blood sugar takes a long time to go back to normal, you could have diabetes.

You might see a measurement from the test written out as “mg/dL.” It stands for milligrams per deciliter. 

Glucose tolerance test normal range

The normal range for blood glucose is 139 mg/dL or lower. 

If your level is between 140 mg/dL and 199 mg/dL, that's a sign your glucose tolerance is impaired. This is also called prediabetes. You're at risk of having type 2 diabetes. Your risk of heart disease is also elevated, even if you never have diabetes.

If your blood glucose level is 200 mg/dL or higher, you may have diabetes.

The normal range if you're not pregnant can be broken down further: 

A normal fasting level is 60 to 99 mg/dL. 

At the 1-hour mark, a normal level is less than 200 mg/Dl. 

Glucose challenge test normal range

If you're pregnant and having the 1-hour glucose challenge test to check for gestational diabetes, a normal reading is below 140 mg/dL.

If your blood glucose level after the 1-hour test is higher than 199 mg/dL, you have gestational diabetes. 

For the the 3-hour test to check for gestational diabetes, here are the ranges: 

  • A normal fasting blood glucose level is lower than 95 mg/dL.
  • At the 1-hour mark, a normal level is lower than 180 mg/dL. 
  • At the 2-hour mark, a normal level is lower than 155 mg/dL. 
  • At the 3-hour mark, a normal level is lower than 140 mg/dL. 

If one of your results is higher than normal, your doctor will have you repeat the test in a few weeks. If you have two (or more) readings above normal, you have gestational diabetes.

 

If you have prediabetes, your doctor will talk to you about ways to stop it from turning into a full-blown case. Exercise and weight loss are two ways to lower your risk of type 2 diabetes.

If the test shows you have diabetes, you might get what’s called an A1c or other tests to confirm the diagnosis. Diet, exercise, and medicine can help control your blood sugar.

Good foods and physical activity can also help control diabetes during pregnancy. Your blood sugar should go back to normal after your baby is born.

But gestational diabetes increases your risk of getting type 2 diabetes after your pregnancy. You'll need to stay on a healthy diet and exercise plan to avoid a future diabetes diagnosis.

An oral glucose tolerance test – also called OGTT – checks how your body processes sugar. Your results might show that you're at risk of getting type 2 diabetes, or that you already have it. If you're pregnant, you'll have your glucose tolerance tested to make sure you don't have gestational diabetes. For the test, you fast for 8 hours and then have blood drawn to establish your fasting glucose level. Depending on the type of test you're taking, you'll have blood drawn again after 1 hour, 2 hours, or 3 hours. The risks of the tests are minimal, and you can go back to your normal routine right afterward. 

How many hours is OGTT fasting?

Your last meal should be at least 8 hours before your test. You may want to schedule it first thing in the morning so that you fast overnight. You're allowed water, but no other liquids. 

How do I pass the OGTT?

A normal blood glucose reading in this test is 139 mg/dL or lower. 

What's the difference between OGTT and GTT?

The oral glucose tolerance test (OGTT) is sometimes called a glucose tolerance test (GTT). The shorter, 1-hour version of the test is called the glucose challenge test.