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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 examines what an oral glucose tolerance test is, how it works, how it differs from 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 (g) of glucose:

DiagnosisResult
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.

The doctor may provide a person with a glucose drink or prescription, and then they have to present it at the pharmacy or laboratory. Those who need to fill a prescription may have to call their pharmacist ahead of time to check if glucose drinks are in stock.

Researchers suggest eating at least 150 g of carbohydrates for 3 days before the test, such as pasta, rice, crackers, and bread.

A person should also avoid eating, smoking, or engaging in physical exercise for a minimum of 8 hours before the test. This means that if their appointment is at 8 a.m., they cannot eat or drink after midnight the day before. People can drink non-flavored still water before the test.

Individuals should not drink their glucose drink without the doctor’s approval. On the day of the OGTT, a healthcare professional performs a fasting blood glucose test. Then, they do another test after the person drinks the glucose drink.

People should inform the doctor of any medications they are taking. The healthcare professional may recommend not taking medication before the test.

During pregnancy, the hormones from the placenta responsible for fetal growth may cause the cells to become resistant to insulin. The Royal Women’s Hospital explains that when a person is pregnant, their body produces more insulin, but if this does not happen, gestational diabetes may develop.

A healthcare professional will perform an OGTT on people who are pregnant to check whether they have gestational diabetes. According to the University of Michigan Health System (UMHS), they can do the test on its own or as a second test in a two-part screening.

The American College of Obstetricians and Gynecologists recommends that a person has a test for gestational diabetes at 24–28th weeks of gestation.

The NIDDK affirms that a medical doctor may have to draw blood every hour for 2–3 hours to test for gestational diabetes. In addition, if two or more readings indicate that the individual has high blood sugar levels, this may mean that they have gestational diabetes.

Furthermore, individuals with a high chance of developing gestational diabetes may have a test during their first prenatal visit. These include those who have:

In addition, the United Kingdom’s National Health Service (NHS) states that a person may have to use a blood sugar test kit to monitor their effects during pregnancy. They can also engage in physical exercise and follow a healthy diet to help reduce their blood sugar levels. A person may also have to take medication to control their blood sugar.

Doctors may also start a person’s labor earlier than their due date if there are any health complications.

The UMHS also states that some individuals may have low blood sugar levels at the end of the test. Symptoms may include:

  • hunger
  • sweating
  • feeling nervous
  • restlessness

In one 2016 study, researchers found that adverse effects may occur in people who have undergone bariatric surgery and take an OGTT. Some adverse effects include:

Diabetes screening tests cost approximately $20–70. There may also be visit fees that can range from $100–200.

Health insurance companies may be able to cover some or all the fees of the OGTT, but it is best to contact the insurance provider directly and check what they offer.

Those uninsured can check look for independent labs, as their prices are usually lower than a doctor’s office or hospital.

Further to that, some labs may also have a patient assistance program. This may allow for monthly payments and may suit those who need regular tests.

If a person does not have health insurance through work 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 OGTTs.

Pros

  • gestational diabetes screening during pregnancy
  • most accurate test for pregnant people
  • patient assistance programs may be available
  • performed by a healthcare professional, so less chance for inaccuracy
  • identifies patients with diabetes and reduces fetal and maternal diseases

Cons

  • possible adverse effects in people who have had bariatric surgery
  • fasting for at least 8 hours
  • no exercise before the test
  • duration of up to 3 hours

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

Fasting plasma glucose test

A fasting plasma glucose test screens for blood sugar 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 sugar 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 A1C home diabetes tests available to purchase online, including:

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.

An A1C test can also be performed at a doctor’s office with less time to wait for results. This test may include a blood draw or a finger prick.

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.

DiagnosisResult
NormalBelow 5.7%
Prediabetes5.7–6.4%
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) state 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 common questions about OGTTs.

How long does a test take?

An OGTT takes up to 3 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 is the normal range for an OGTT?

According to the American Diabetes Association, a normal result for an OGTT is less than 140 mg/dl.

If a person has prediabetes, the OGTT range is 140–199 mg/dl, and in the case of diabetes, 200 mg/dl or more.

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 or insulin 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. People need to fast before the test. Healthcare professionals will then ask them to drink a liquid that contains glucose and undergo a blood test to check their blood sugar levels.

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