The oral glucose tolerance test (OGTT) or glucose tolerance test (GTT) can check for prediabetes or diabetes. A person drinks a liquid containing glucose. The doctor then checks their blood to see how their body reacts.

Diabetes is a group of conditions in which the body cannot process glucose effectively because of insulin deficiency or resistance to insulin at the cellular level. This results in high blood sugar levels.

Diabetes is the eighth leading cause of death in the United States, according to the Centers for Disease Control and Prevention (CDC). In addition, up to 10% of pregnant people each year develop gestational diabetes, which usually resolves after pregnancy.

Early diagnosis can be the key to effective treatment and reducing the risk of long-term complications. Doctors commonly use the glucose challenge test or the oral glucose tolerance test (OGTT) to diagnose diabetes.

This article discusses what these tests are, when doctors do them, what the results mean, any risks of the tests, and other ways to confirm diabetes.

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Some people may refer to this type of test as the glucose challenge test or the OGTT. However, those are actually two separate types of test. Another name for the challenge test is the glucose screening test.

For either test, a person must drink a sugary drink, and a healthcare professional then performs a blood test to determine how much glucose is in the person’s blood. However, the OGTT is more complex than the challenge test and can be more expensive.

Healthcare professionals often perform both types of tests for pregnant people but may also choose to use them for nonpregnant people.

Both the OGTT and the challenge test measure glucose levels in the body.

A healthcare professional will measure and compare blood glucose levels before and after a person consumes a sugary drink. This helps them see how the person’s body processes glucose.

The challenge test usually takes an hour, whereas the OGTT takes 2–3 hours.

If a person does not have diabetes, their blood glucose level will rise after they eat sugary foods and return to normal after their body absorbs the glucose. In a person with diabetes, blood sugar levels may remain high. These tests measure this response.

The following chart summarizes typical and atypical blood glucose levels under various conditions. More detail follows below.

normalless than 140 mg/dL
prediabetes140–199 mg/dL
diabetes200 mg/dL or greater

The test results can indicate whether a person’s blood glucose falls within the normal, prediabetes, or diabetes range.

People may need the test if they are at risk of developing diabetes or have symptoms that may indicate diabetes.

Pregnant people usually receive the challenge test first as a step toward ruling out gestational diabetes. For a person who is at low risk of gestational diabetes, the doctor will usually perform the test at 24–28 weeks of gestation.

In cases of higher risk, a doctor may perform the test earlier. If the test results are above the normal range, a person will usually undergo the OGTT to confirm.

Doctors may use the challenge test and OGTT in nonpregnant people. In some cases, they may prefer to administer the fasting plasma glucose test, which is less expensive and easier than the OGTT.

A note about sex and gender

Sex and gender exist on spectrums. This article will use the terms “male,” “female,” or both to refer to sex assigned at birth. Click here to learn more.

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The test may vary a bit depending on whether a person is pregnant.

In pregnant people

Pregnant people may undergo both the glucose challenge test and the OGTT. The procedure is as follows:

  • Glucose challenge or screening: The person has a blood test without fasting, drinks a glucose drink, and has another blood test 1 hour later. If the result is 135–140 mg/dL or higher, the doctor may want to do the OGTT.
  • OGTT: The person has a fasting blood test, drinks a glucose drink, and has further blood tests 1, 2, and maybe 3 hours later.

The National Institute of Diabetes and Digestive and Kidney Diseases confirms that a doctor may need to draw blood every hour for 2–3 hours to check for gestational diabetes.

If two or more readings indicate that the person has high blood sugar levels, they may have gestational diabetes.

In nonpregnant people

A person is most likely to have the challenge test if they are pregnant. A nonpregnant person will usually undergo the OGTT. They will need to fast for at least 8 hours before the test.

The person should let their doctor know in advance about:

  • any regular medications they are taking
  • any exercise regimens they follow
  • any other health conditions they have

Researchers suggest that a person eat their typical diet or one that contains at least 150 grams of carbohydrates per day — such as pasta, rice, crackers, and bread — for 3 days before the test.

The doctor may advise about how to take any regular medications during the fasting period.

On the day of the test, a healthcare professional will:

  • take a blood sample before the test begins
  • ask the person to consume a sugary drink containing glucose and water
  • take blood samples 30, 60, 90, and 120 minutes after a person consumes the drink.


The test result will indicate whether a person has diabetes or is at risk of developing it. If the test result suggests prediabetes or diabetes, a doctor will discuss treatment options.

Some medications and other factors can lead to high blood glucose levels. Sometimes, a doctor will do a different test or repeat the test to confirm the results.

During pregnancy, hormones can prevent the body from using insulin effectively. This is called insulin resistance.

Many people still produce enough insulin to overcome the resistance, but some do not. In some cases, insulin resistance progresses to gestational diabetes.

Gestational diabetes can lead to the following complications:

  • high blood glucose in the fetus and low levels after birth
  • difficulties during labor and the need for a cesarean delivery
  • higher risk of vaginal tearing during delivery and bleeding afterward
  • risk of developing type 2 diabetes in the future

The American College of Obstetricians and Gynecologists recommends that a person undergo the 1-hour glucose challenge test for gestational diabetes at 24–28 weeks of gestation. If the results indicate the person may have diabetes, they will then need to undergo the 3-hour tolerance test to confirm.

Who is at risk?

Risk factors for gestational diabetes include:

  • occurrence of gestational diabetes in an earlier pregnancy
  • family history of diabetes
  • being overweight and other diabetes-related conditions

These individuals have a high chance of developing gestational diabetes and may have a test during their first prenatal visit.

Other risk factors include those who:


If test results suggest that a person’s blood sugar levels are high, a doctor may recommend that the person:

  • follow a healthy diet that is suitable for gestational diabetes
  • get enough exercise
  • monitor their blood sugar levels
  • have more frequent prenatal care visits
  • use supplemental insulin, in some cases

Healthcare professionals will advise on each person’s needs and treatment plan because diabetes affects everyone differently.

A blood test poses very few risks. A healthcare professional uses a needle to draw the blood, so the injection may cause moderate pain for some people. Afterward, a person may experience only slight bruising in the area where the needle entered the skin.

Because they involve fasting and blood testing, glucose screening or tolerance tests may cause some lightheadedness. A person may want to come to the testing clinic with someone who can take them home.

More serious but less common risks include:

  • excessive bleeding
  • fainting
  • blood accumulation under the skin
  • infection

Some individuals may also have low blood sugar levels during or after the test. In a 2021 study, researchers defined this as at least one glucose value below 60 mg/dL. They found that about 10.4% of study participants developed hypoglycemia during the test.

The symptoms of hypoglycemia may include:

  • hunger
  • sweating
  • nervousness
  • restlessness

Adverse effects, including hypoglycemia and unstable blood sugar levels, may also occur in people who have undergone bariatric surgery and do a glucose tolerance test. Researchers recommend avoiding this test in pregnant people who have had this surgery.

It is difficult to determine a price of these tests, as it can vary widely based on the laboratory, doctor’s office, or hospital performing the procedure.

Health insurance companies may cover some or all of the cost, but it is best for a person to contact their insurance provider directly and check what they offer.

People who do not have insurance can look for independent labs, which may have lower prices than a doctor’s office or hospital.

Some labs may also have a patient assistance program that allows monthly payments and suits those needing regular tests.

If a person does not have health insurance through an employer or the marketplace, they may qualify for Medicaid, depending on their state policy and income. They may also be able to receive care from a federally qualified health center, which provides care to all individuals regardless of insurance status or ability to pay.

There are advantages and disadvantages of the challenge test or the OGTT:


  • gestational diabetes diagnosis during pregnancy
  • most accurate test for pregnant people
  • may be more affordable with help from patient assistance programs
  • low chance of inaccuracy because a healthcare professional performs the test
  • helps reduce potential complications for pregnant people and babies


  • possible adverse effects in people who have had bariatric surgery
  • may require fasting for at least 8 hours
  • may require additional preparations
  • duration of up to 3 hours in some cases
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The glucose tolerance test is not the only way to diagnose diabetes. Doctors also use other tests to diagnose and monitor the condition.

Hemoglobin A1C

This test measures a person’s average blood glucose over the past 3 months. It shows the percentage of blood sugar attached to hemoglobin, the protein that carries oxygen in the red blood cells.

A normal level is less than 5.7%. Levels of 5.7–6.4% suggest prediabetes, and 6.5% and above indicate diabetes.

People do not need to fast before taking this test. However, this test is not accurate in people with anemia.

A doctor may use a different test if a person is African American or of Mediterranean or Southeast Asian descent, as these individuals may have the following hemoglobin variants:

  • Hemoglobin S: African American people are more likely to have the variant hemoglobin S, which can cause sickle cell disease.
  • Hemoglobin C: People who are African American or of South and Central American, Caribbean, or European descent are more likely to have the variant hemoglobin C, which can cause hemoglobin C disease.
  • Hemoglobin E: Asian Americans, especially those of Southeast Asian descent, are more likely to have the variant hemoglobin E, which can cause hemoglobin E disease.

People with these hemoglobin variants may receive a test result that is falsely high or low. This may lead to doctors withholding treatment or providing ineffective therapies for diabetes, which may, in turn, result in complications.

Fasting plasma glucose

This test measures blood glucose levels while a person is fasting. The person cannot eat or drink anything, except sips of water, for at least 8 hours beforehand.

Glucose levels of 126 mg/dL or higher indicate diabetes. Prediabetes levels are 100–125 mg/dL, and normal levels are below 100 mg/dL.

Random blood sugar test

For this test, a healthcare professional can take a blood sample at any time, not necessarily when a person is fasting. People who have serious diabetes symptoms may undergo this test. A blood glucose level of 200 mg/dL at any time indicates diabetes.

People with diabetes should monitor their blood glucose levels regularly with a home testing kit or continuous glucose monitor.

A person cannot do the glucose challenge test or the OGTT at home. However, various home blood glucose monitoring tests are available for purchase online.

These allow people to monitor their blood sugar levels and provide data points they can discuss with their doctor.

Additionally, glucose tolerance test and home test results will appear differently. The following table shows the results a person may receive after an A1C test.

normalbelow 5.7%
diabetes6.5% and above

A person should speak with their doctor before using home diabetes tests to ensure this is the best approach and to understand how to handle the results.

The CDC states that a person should contact a doctor if they have the following symptoms of diabetes:

  • very frequent urination, particularly at night
  • excessive thirst or hunger
  • unintentional weight loss
  • blurry vision
  • numbness or tingling in the hands or feet
  • excessive tiredness
  • dry skin
  • slow wound healing
  • frequent infections

It is important to contact a doctor as soon as possible if a person is experiencing any symptoms of diabetes. Without treatment, diabetes can lead to complications such as diabetic retinopathy, nerve damage, and heart attack.

Below are answers to some common questions about the glucose tolerance test.

How long does a test take?

The test can last up to 3 hours. In pregnancy, a person may do the test in two parts. The first phase is a 1-hour test. If the test indicates that they may have diabetes, the person must do a longer 3-hour test.

In either case, a person will drink a liquid containing glucose, and then a healthcare professional will draw blood to determine how much glucose is in the blood.

What is the normal range for a glucose tolerance test?

According to the American Diabetes Association, a normal result is less than 140 mg/dL 2 hours after a person consumes a sugary drink.

If a person has prediabetes, the range is 140–199 mg/dL. And in the case of diabetes, it is 200 mg/dL or more.

What medication may a doctor prescribe?

If a doctor diagnoses diabetes, they may prescribe insulin. They will recommend insulin for people with type 1 diabetes and some people with type 2 diabetes. This medication can help manage people’s blood sugar levels.

Doctors may prescribe metformin (Fortamet, Glumetza) or glyburide (Amaryl) for people with type 2 diabetes or gestational diabetes. However, researchers are still investigating whether these medications are safe during pregnancy.

In cases of gestational diabetes, a person will also have to eat a nutritious diet and monitor their blood sugar levels.

A pregnant person should ask their doctor about the appropriate weight gain for their pregnancy. When a person has gestational diabetes, gaining too much weight or gaining it too soon can make blood sugar levels harder to manage.

Following the Mediterranean diet can be a good option for many people, but a person should consult with their doctor to determine the best dietary plan for them and their baby.

Learn more about how to manage gestational diabetes.

Glucose tolerance testing is an important tool for diagnosing diabetes. If a person’s blood glucose levels are above 140 mg/dL, this may indicate prediabetes or diabetes.

The U.S. Preventive Services Task Force recommends regular screening for people aged 35–70 and for younger people who have risk factors such as obesity, previous gestational diabetes, or a family history of diabetes.