Glucose tolerance test: What you should know

Glucose tolerance test: What you should know

The glucose tolerance test is a blood test, also known as diabetes, for diagnosing diabetes mellitus. How the body responds to glucose is seen in the test.

Diabetes is a chronic disease of the blood in which, due to insulin deficiency or resistance to insulin within cells, the body can not efficiently absorb glucose. This results in high sugar levels in the blood.

According to the Centers for Disease Control and Prevention ( CDC), diabetes is the seventh leading cause of death in the United States.

The secret to successful treatment and reducing the risk of long-term complications could be early diagnosis. The oral glucose tolerance test ( OGTT) is the most common type.

Find more about the test for glucose tolerance, any procedure complications, and other ways to confirm diabetes.

What is the test?

The glucose tolerance test can help diagnose diabetes.
The glucose tolerance test can help diagnose diabetes.

A glucose tolerance test measures glucose levels in the body.

Before and after a sugary drink, a healthcare provider can test and compare the blood glucose levels of an individual.

Because an individual conducts the test over 2 hours, it will show the doctor how glucose is absorbed by the body.

The blood glucose level in a healthy person will increase after consuming sugary food and return to normal after the glucose is absorbed by the body. Blood sugar levels can remain high in a person with diabetes.

The test measures this response.

What to expect

An individual should fast for 8–12 hours before taking the test. They do not eat or drink, but during this time, they can normally sip some water.

In advance, the person should talk to their doctor about:

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

The doctor may give advice about how to take any regular medications during the time of fasting.

On the day

On the day, the healthcare provider will:

  • take a blood sample before the test begins
  • ask the person to consume a sugary drink containing glucose and water
  • take further blood samples every 30–60 minutes for a total of 2 hours


The final result will indicate whether a person has or is at risk of developing diabetes:

  • Normal: Below 140 milligrams per deciliter (mg/dL)
  • Prediabetes: 140–199 mg/dL
  • Diabetes: 200 mg/dL or above

If the test result suggests prediabetes or diabetes, a doctor will discuss treatment options for addressing this.

Various factors can alter the accuracy of the test.

For reliable results, a person must:

  • have relatively stable health
  • be managing any other health conditions effectively

Some medications and other factors can lead to high blood glucose levels.

Sometimes, a person will do a different test or repeat the test, to confirm the results.

Glucose tolerance during pregnancy

The test for glucose tolerance may also assist in diagnosing gestational diabetes.

The person may take a two-part test:

Glucose screening test: Without fasting, the person has a blood test, then drinks a glucose drink and one hour later has another blood test. The doctor may prescribe a second test, the glucose tolerance test, if the result is 140 mg / dL.

Glucose tolerance test: The person will have a fasting blood test, then drink a glucose drink, and have 1, 2, and maybe 3 hours later, additional blood tests.

If the level of blood sugar is high and the person has not been diagnosed with diabetes before, it is possible that the doctor will diagnose gestational diabetes.

What is gestational diabetes?

Women may need to control their blood sugar levels during pregnancy on their own.
When the body is unable to produce all of the insulin it requires for pregnancy, gestational diabetes begins.

Low levels of insulin, combined with hormonal changes, can lead to resistance to insulin. When this happens, high levels of glucose build up in the blood.

This 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 cesarian delivery
  • a higher risk of vaginal tearing during the birth and bleeding after delivery
  • a risk of developing type 2 diabetes in the future

A glucose tolerance test is typically prescribed by doctors between weeks 24–28 of pregnancy. Earlier in the pregnancy, those with a higher risk may need a test.

Who is at risk?

Risk factors for gestational diabetes include:

  • having had gestational diabetes in an earlier pregnancy
  • a family history of diabetes
  • having obesity or other diabetes-related conditions
  • high blood pressure
  • being physically inactive
  • older age

According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), if a person gains more weight than normal during pregnancy, this could be a sign of gestational diabetes.

A research released in 2015 states that 14% of pregnancies are affected by gestational diabetes every year.


a Pregnant lady preparing low-sugar diet
Whether diabetes occurs during pregnancy or not, a healthful, low-sugar diet is essential.

If results suggest that blood sugar levels are high, the doctor may recommend the following:

  • following a healthful diet that is suitable for gestational diabetes
  • getting enough exercise
  • monitoring blood sugar levels
  • attending more frequent screening and seeking medical help if glucose levels go up
  • insulin use, in some cases

The healthcare professional will advise on the needs and treatment plan of each person, because diabetes affects everybody differently.

Risks and side effects

No side effects from glucose testing are encountered by most people, and severe complications are uncommon.

The glucose tolerance test could cause nausea, light-headedness, shortness of breath, and sweating in some individuals because it involves fasting and blood testing.

The doctor uses a needle to draw blood, so some can feel mild pain from the injection.

Risks that are more extreme but less frequent include:

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

Other diabetes tests

The glucose tolerance test is not the only method to diagnose diabetes. For diagnosis and monitoring of the condition, physicians use other tests.

Hemoglobin A1C

Average blood glucose over 2–3 months is assessed by this test. It indicates the percentage of blood sugar in the red blood cells that is attached to hemoglobin, the protein that carries oxygen. Normal levels are 5.6 percent or below, 5.7-6.4 percent indicates prediabetes, and diabetes is suggested by 6.5 percent and above.

Fasting plasma glucose

While a person is fasting, this test tests blood glucose levels. For at least 8 hours in advance, the person will not be able to eat or drink anything, except sips of water.

Glucose levels greater than or equal to 126 mg / dL indicate diabetes. The levels of prediabetes are 100-125 mg / dL, and the normal concentrations are below 100 mg / dL.

Random blood sugar test

At any time, and not just while fasting, a doctor takes a blood sample. This test may be done for people who have severe symptoms of diabetes. This means that diabetes is present if blood glucose levels are 200 mg / dL at any time.

People with diabetes should use a home test kit or continuous glucose monitor to monitor their blood glucose levels periodically.


A significant diagnostic method for diabetes identification is glucose tolerance testing. This can suggest diabetes if blood glucose levels are above 140 mm / dL.

For those with risk factors , such as obesity, previous gestational diabetes, or a family history of diabetes, Current guidelines recommend regular screening for individuals from the age of 45 years or younger.