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An oral glucose tolerance test is one method doctors use to screen for type 2 diabetes and prediabetes. These tests involve drinking a product that contains glucose and drawing blood to check how much glucose it has.

This article looks at what an oral glucose tolerance test is, how it works, the difference between this test and a home diabetes test, and when to contact a doctor.

An oral glucose tolerance test (OGTT) tests for type 2 diabetes and insulin resistance. It informs health professionals whether a person can metabolize sugars and carbohydrates.

An OGTT can indicate whether an individual has the following conditions:

Learn more about the different types of diabetes.

The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) states that a doctor will usually only perform an OGTT to test for gestational diabetes. Healthcare professionals may use this test to screen for prediabetes and type 2 diabetes. However, an OGTT is more expensive than other diabetes tests and more difficult to conduct.

An OGTT requires a person to fast for at least 8 hours. On arrival at a health clinic, a healthcare professional will take a blood sample. The person then drinks a product containing glucose, after which a healthcare professional takes another blood sample after 2 hours.

The results of an OGTT can indicate whether an individual has prediabetes or diabetes. The following table shows test results 2 hours after drinking a liquid that contains 75 grams of glucose:

Normal139 mg/dl or below
Prediabetes140–199 mg/dl
Diabetes200 mg/dl or above

Healthcare professionals will give pregnant people drinks that contain more glucose and use different values to diagnose gestational diabetes.

There are several tests that healthcare professionals may use to diagnose diabetes.

Fasting plasma glucose test

A fasting plasma glucose test screens for blood glucose levels at a single point in time. People will need to fast for 8 hours before taking this test.

Random plasma glucose test

A random plasma glucose test does not require fasting. A healthcare professional may use this test when an individual has symptoms of diabetes and doctors do not want to wait to perform the screening.

A1C test

An A1C test screens for average levels of blood glucose over 3-months. People do not need to fast before taking this test.

However, this test is not accurate in people with anemia. A doctor may also 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 Americans are more likely to have the hemoglobin 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 hemoglobin 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 hemoglobin variant hemoglobin E, which can cause hemoglobin E disease.

People with these hemoglobin variants may return 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.

A person cannot have an OGTT at home. However, there are a variety of home diabetes tests available to purchase online, including:

  • LetsGetChecked Diabetes Test: This finger-prick test measures the percentage of glucose present in the blood. The company recommends people with diabetes or symptoms of diabetes take this test to monitor their blood sugar levels.
  • EverlyWell Monitor Your Blood Sugar Levels: This finger-prick test monitors a person’s blood sugar levels over the last 90 days. The company states it may help an individual and their doctor check the effectiveness of their diabetes treatments.
  • Imaware At Home HbA1c Test Kit for Diabetes: This finger-prick test also monitors people’s blood sugar levels. The company states that a person needs to fast for 8 hours before taking the test.

However, there are differences between an OGTT and a home diabetes test.

At-home diabetes tests allow people to monitor their blood sugar levels and indicate when to contact a doctor for a diagnosis. An OGTT is only available from a healthcare professional and can immediately indicate prediabetes or diabetes.

A person should only receive an OGTT from a healthcare professional, whereas an individual can take a home diabetes test anytime and anywhere. While both tests require a blood sample, home diabetes tests require less blood as it is a finger-prick test.

Additionally, OGTT and home diabetes test results will appear differently. Home diabetes tests are A1C tests, which present results in an alternate way. The following table shows the results a person may receive after an A1C test.

NormalBelow 5.7%
Diabetes6.5% and above

People who take home diabetes tests should contact a doctor after receiving results that indicate they may have prediabetes or diabetes. A healthcare professional may perform additional tests and advise people on how to manage their condition.

The Centers for Disease Control and Prevention (CDC) states a person should contact a doctor if they have the following symptoms of diabetes:

  • urinating very frequently, particularly at night
  • feeling very thirsty or hungry
  • experiencing unintentional weight loss
  • having blurry vision
  • experiencing numb or tingling sensations in hands or feet
  • feeling tired
  • experiencing dry skin
  • having slow healing sores
  • contracting 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, heart attack, and cancer.

Below are some frequently asked questions about OGTTs.

How long does a test take?

An OGTT takes around 2 hours. A person will drink a liquid that contains glucose, after which a doctor will draw blood 2 hours later to see how much glucose is present in the blood.

What medication may a doctor prescribe?

If a healthcare professional diagnoses an individual with diabetes, they may prescribe insulin. They will recommend insulin for people with type 1 diabetes and some with type 2 diabetes. This medication can help control blood sugar levels.

Learn more about insulin.

Doctors may prescribe metformin for people with type 2 diabetes or gestational diabetes. This medication reduces the amount of glucose the liver releases into the body.

Learn more about metformin.

Some people with type 2 diabetes may manage their condition by maintaining a moderate weight and eating a nutritious, balanced diet. A doctor will work with each individual to find the best treatment plan.

Learn more about how to manage diabetes.

OGTTs can indicate whether a person has prediabetes and diabetes. Before the test, people need to fast. Healthcare professionals will then ask them to drink a liquid that contains glucose and undergo a blood test to check their blood sugar.

While people cannot take an OGTT at home, several at-home diabetes tests are available to buy online. However, individuals should discuss their results with their doctor to receive a diagnosis and discuss treatment plans.