An oral glucose tolerance test (OGTT) measures how well your body processes glucose, or blood sugar. Glucose is your body’s primary source of energy.

Often, an OGTT is used to diagnose prediabetes and diabetes, especially when gestational diabetes is a concern.

It may also be used to test:

  • reactive hypoglycemia
  • acromegaly, a hormonal condition that causes excess growth
  • impaired beta-cell function
  • insulin resistance

An OGTT is a type of blood test. It requires taking several blood samples over a set amount of time.

Before doing an OGTT, a doctor might perform a glucose challenge test first. This is a shortened version of the OGTT.

A glucose challenge test requires no fasting. During the test, you’ll drink a beverage containing 50 grams of glucose. After 1 hour, a healthcare professional will take a blood sample to check your blood sugar level.

If your blood glucose is higher than 140 mg/dL, it might indicate diabetes. In this case, you’ll need an OGTT.

An OGTT requires some preparation. You’ll need to fast for about 8 hours before the test is performed. This means you can’t eat breakfast or drink any liquids, except water, beforehand.

An OGTT includes the following steps:

  • A health worker will take a blood sample from your fingertip, earlobe, or a vein. They’ll test the sample for blood glucose, which will serve as a baseline measurement.
  • You’ll drink a concentrated glucose beverage. Most solutions contain 75 grams of glucose.
  • You’ll sit or lay down for 1 hour.
  • After 1 hour, healthcare staff will take a blood sample.
  • A healthcare professional may take another blood sample after 2 hours and again at 3 hours.
  • A medical team will measure the blood glucose levels at each test time.

Between testing times, you’ll need to stay still and avoid drinking a lot of water. That’s because excessive movement and hydration can alter the results.

You might receive an OGTT without receiving a glucose challenge test first.

An OGTT must be ordered by an advanced nurse practitioner or doctor. This may include your primary care physician, gynecologist, or endocrinologist.

The shortened OGTT, or glucose challenge test, is also part of prenatal care. It’s done as a basic screening test for pregnant people.

An OGTT can be performed in the following settings:

  • a doctor’s office
  • a clinical laboratory
  • an outpatient clinic at a hospital

Depending on the setting, the following professionals may do the test:

  • a medical assistant
  • a phlebotomist
  • a nurse

What if I don’t have health insurance?

If you don’t have a doctor or health insurance, there are clinics that offer free or low-cost prenatal care.

Can you order an oral glucose tolerance test online?

There are no at-home OGTT kits for sale. It’s safer to complete this test under medical supervision. Depending on your health conditions, drinking liquid containing a large amount of sugar at home may be unsafe.

However, you can order at-home blood glucose monitors and hemoglobin A1c (HbA1c) tests. These diabetes tests may help manage diabetes or determine your risk. For best results, ask a doctor if you need these diabetes home tests.

An OGTT can be used to effectively diagnose various types of diabetes, including prediabetes.

It’s useful for confirming a diagnosis after an abnormal HbA1c test result. An A1C test is often part of routine blood tests, but it’s less sensitive than an OGTT. Because of this, an OGTT can help with making an official diagnosis.

A 2019 study also found that an OGTT is a more effective screening tool than an A1C test. Similarly, a 2020 study determined than an OGTT can be used to diagnose diabetes at earlier stages than the A1C test can. This means management and treatment can begin sooner rather than later.

But, like most clinical tests, an OGTT has some limitations. The test can be easily influenced by lifestyle factors such as:

  • physical stress
  • severe psychological stress
  • exercise
  • illness
  • recent surgery

This can alter the results, potentially leading to a misdiagnosis.

An OGTT is also time consuming. You may need to take off from work or school. There’s currently no alternative for the test, according to 2018 research.

An OGTT is considered to be a safe test.

Be sure to eat a meal the night before. This may help reduce discomfort caused by fasting.

For some people, drinking the concentrated glucose solution causes side effects such as:

  • nausea
  • sweating
  • vomiting (less common)

Additional side effects, such as diarrhea and heart palpitations, are more likely to affect people who’ve had bariatric surgery.

Other OGTT risks are the same as getting your blood drawn for any reason:

  • pain
  • bleeding
  • bruising
  • soreness
  • rash
  • skin irritation from adhesive on a bandage

If you have any concerns, talk with a doctor before taking the test.

The next steps depend on your results and overall health.

After your blood samples have been tested by a laboratory, a healthcare professional will send you the results. They’ll indicate what your OGTT results mean.

If your blood glucose levels are normal, it means you don’t have diabetes. You won’t have to do anything else except continue following healthy lifestyle habits, such as eating a balanced diet and exercising. A doctor can let you know when you should have your next routine blood test.

If your blood glucose levels are high, you may receive a diabetes diagnosis. A healthcare team can develop a diabetes management plan to help reduce complications.

You may be asked to:

  • take diabetes medication
  • exercise more often
  • make changes to your diet
  • use a blood glucose meter to monitor your blood glucose
  • visit an endocrinologist

If you’re pregnant and have a diagnosis of gestational diabetes, you may need to have an OGTT 4 to 12 weeks after giving birth. This will check to see if you’ve developed type 2 diabetes.

If your results from this test are normal, you may be asked to get an OGTT every 1 to 3 years as a routine checkup.

When is an oral glucose tolerance test done?

An OGTT is done when a healthcare professional thinks you might have diabetes, especially gestational diabetes.

Typically, it’s ordered after an abnormal HbA1c test or glucose challenge test. These tests also check how well your body manages sugar, but they’re less sensitive than an OGTT. Thus, the OGTT is more effective for diagnosis.

A healthcare professional might also order an OGTT if they think you have another condition that affects glucose metabolism.

Can I do an oral glucose tolerance test at home?

At this time, it’s not possible to perform an OGTT at home. That’s because a healthcare professional needs to monitor you during the test. For some people, it may be unsafe to drink liquid containing a large amount of sugar, so it’s important to do so under medical supervision.

Do I need an oral glucose tolerance test?

You might need an OGTT if your HbA1c level is high. A higher A1C level indicates prediabetes (5.7 to 6.4 percent) or diabetes (6.5 percent or higher).

If you’re pregnant, you’ll also need an OGTT to screen for gestational diabetes between 24 to 28 weeks. Typically, this includes the nonfasting, 1-hour test. If the results are abnormal, you’ll need to do a 2- or 3-hour OGTT.

If you’re at risk of gestational diabetes, you might need an OGTT before 24 weeks. Risk factors include:

How do I know if I need an oral glucose tolerance test?

A doctor can let you know if you need an OGTT. They’ll order the test if you’re at risk of diabetes or have had abnormal screening results for diabetes.

An OGTT measures how well your body manages glucose. It’s often used to diagnose diabetes, including gestational diabetes. The test is typically done 24 to 48 weeks into pregnancy.

During the test, you’ll need to drink a concentrated glucose solution. Your blood will be drawn after 1, 2, and 3 hours. A lab will then measure the glucose levels in your blood samples.

If a doctor has ordered an OGTT, be sure to follow their instructions. You might need to fast beforehand. The test may be time consuming, but it’s a crucial step in diagnosing and managing diabetes.