The glucose tolerance test is a blood test for diagnosing diabetes mellitus, also known as diabetes. The test shows how the body reacts to glucose.
Diabetes is a chronic blood condition in which the body cannot process glucose effectively because of insulin deficiency or resistance to insulin within cells. This results in high blood sugar levels.
Diabetes is the seventh leading cause of death in the United States, according to the Centers for Disease Control and Prevention (CDC).
Early diagnosis can be the key to effective treatment and reducing the risk of long-term complications. The most common type is the oral glucose tolerance test (OGTT).
Find out about the glucose tolerance test, any risks of the procedure, and other ways to confirm diabetes.
A glucose tolerance test measures glucose levels in the body.
A healthcare provider will measure and compare a person’s blood glucose levels before and after a sugary drink.
Because a person does the test over 2 hours, it can show the doctor how the body processes glucose.
In a healthy person, the blood glucose level will rise after eating sugary food and return to normal after the body absorbs the glucose. In a person with diabetes, blood sugar levels may remain high.
The test measures this response.
Before taking the test, a person should fast for 8–12 hours. They cannot eat or drink, but they can usually sip some water during this time.
The person should speak to their doctor in advance 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.
The glucose tolerance test can also help diagnose gestational diabetes.
The person may take a two-part test:
Glucose screening test: The person has a blood test without fasting, then drinks a glucose drink and has another blood test one hour later. If the result is 140 mg/dL, the doctor may recommend a second test, the glucose tolerance test.
Glucose tolerance test: The person will have a fasting blood test, then drink a glucose drink and have further blood tests 1, 2, and maybe 3 hours later.
If blood sugar levels are high, and the person has not had a diagnosis of diabetes before, the doctor will probably diagnose gestational diabetes.
What is gestational diabetes?
Gestational diabetes starts when the body is unable to make all of the insulin it needs for pregnancy.
Low insulin levels, combined with hormonal changes, can lead to insulin resistance. 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
Doctors usually recommend a glucose tolerance test between weeks 24–28 of pregnancy. Those with a higher risk may need a test earlier in the pregnancy.
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
If a person gains more weight than is usual during pregnancy, this may be a sign of gestational diabetes, according to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).
A study published in 2015 notes that gestational diabetes affects 14 percent of pregnancies every year.
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 provider will advise on each person’s needs and treatment plan, as diabetes affects everyone differently.
Most people do not experience any side effects from glucose testing, and serious complications are rare.
As it involves fasting and blood testing, the glucose tolerance test might cause nausea, light-headedness, shortness of breath, and sweating in some people.
The doctor uses a needle to draw the blood, so the injection may cause moderate pain for some.
More serious but less common risks include:
- excessive bleeding
- blood collection under the skin
The glucose tolerance test is not the only way to diagnose diabetes. Doctors use other tests to diagnose and monitor the condition.
This test measures average blood glucose over 2–3 months. It shows the percentage of blood sugar attached to hemoglobin, the protein that carries oxygen in the red blood cells. Normal levels are 5.6 percent or lower, 5.7–6.4 percent suggests prediabetes, and 6.5 percent and above indicates diabetes.
Fasting plasma glucose
This test measures blood glucose levels while a person is fasting. The person will not be able to 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
A physician takes a blood sample at any time, and not necessarily when fasting. People who have serious diabetes symptoms may have this test. If blood glucose levels are 200 mg/dL at any time, this indicates that diabetes is present.
People with diabetes should monitor their blood glucose levels regularly, using a home testing kit or continuous glucose monitor.
Glucose tolerance testing is an important diagnostic tool for identifying diabetes. If blood glucose levels are above 140 mm/dL, this may indicate diabetes.
Current guidelines recommend regular screening for people from the age of 45 years, or younger for those with risk factors, such as obesity, previous gestational diabetes, or a family history of diabetes.
When should I receive a glucose tolerance test for the first time?
Males or non-pregnant females may receive an OGTT if fasting blood glucose levels are greater than normal but have not reached levels that suggest diabetes.
An OGTT will then help to confirm if a person has prediabetes or diabetes. For a woman who is pregnant and low-risk for gestational diabetes (GDM), the doctor will usually perform an OGTT at 24-28 weeks of gestational age is when an OGTT is done.
For those found to be at a higher risk for GDM, an OGTT should be done earlier during the pregnancy. Regarding diabetes testing in general, your physician will be able to direct to you the appropriate testing as well as prescribe any changes that you may need to take in your medical care, if needed.
Stacy Sampson, DO Answers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.