In 2015, diabetes was the seventh leading cause of death in the United States.
Early diagnosis can be the key to treating diabetes and reducing the risk of complications in the long run. Find out about this important diagnostic test, any risks of the procedure, and other ways to confirm diabetes.
What is the glucose tolerance test?
The glucose tolerance test can help diagnose diabetes.
An oral glucose tolerance test (OGTT) can help to determine the difference between above normal glucose levels seen in diabetes and prediabetes.
It is a fairly simple test that often is used for confirming the final diagnosis of diabetes in borderline cases.
In a healthy person, the blood glucose level will rise after eating a sugary meal and return to normal once the body takes in the glucose.
The OGTT measures this response to glucose. For this test, the levels of glucose in the blood are compared before and after a sugary drink.
Before taking the test, the patient should fast for at least 8 to 12 hours, but drinking a tiny bit of water during this period is typically fine. A sample of blood will be taken before the test begins. This fasting sample will be used to provide a comparison for the test results.
The doctor will then ask the individual to consume a sugary drink containing a premeasured amount of glucose and water. The exact timing may vary, but another blood sample may be taken every 30 to 60 minutes for a total of 2 hours.
Here is what the different blood glucose levels may indicate by the end of the test:
- Below 140 milligrams per deciliter (mg/dL): A physician would consider this to be normal.
- 140 to 199 mg/dL: This range of results often leads to a diagnosis of impaired glucose tolerance, which is not diabetes but is also not normal, also known as prediabetes.
- Above 199 mg/dL: Levels this high are an indicator of diabetes.
If the test result confirms prediabetes or diabetes, a doctor will discuss treatment options on how to address the medical condition.
Various factors can alter the accuracy of the OGTT. For reliable results, the individual receiving the test must be in relatively stable health with any other underlying conditions under stable control.
The person should also make their physician conducting the test aware of physical exercising habits or any medications taken because heavy exercising and certain medicines could affect their blood glucose level.
Glucose tolerance during pregnancy
The OGTT can also help diagnose gestational diabetes mellitus.
During pregnancy, there are two types of OGTTs available, a one-step and a two-step. The two-step OGTT may require coming in on up to two different days for testing.
Pregnant women who have never had diabetes but demonstrate high blood glucose levels during pregnancy will typically receive a diagnosis of gestational diabetes.
Gestational diabetes starts when the body is unable to make all of the insulin it needs for pregnancy. Without enough insulin available, in addition to hormonal changes of pregnancy that lead to insulin resistance, high levels of glucose build up in the blood.
Untreated or poorly controlled gestational diabetes causes high blood glucose levels to occur in the baby. Then right after birth, the baby could have a significantly low blood glucose level.
Women may need to self-monitor blood sugar levels during pregnancy.
Women generally undergo an OGTT between weeks 24 and 28 of pregnancy. However, women who face an increased risk of diabetes may need a test earlier in the pregnancy.
Risk factors include:
- gestational diabetes in an earlier pregnancy
- a family history of diabetes
- obesity or other medical condition that is associated with a greater risk of developing diabetes
- advanced age
One does not need to fast or change one's diet before the first step part of the two-step test. A woman should not eat or drink anything but sips of water for between 8 and 14 hours before the one-step test or returning for the second step of the two-step test.
The need to fast means this test is least disruptive to have in the morning. Ideally, a friend or family member should take the woman to the test. Their energy levels could be low, or they could experience light-headedness because of not eating.
A blood sample will be taken to get a fasting blood glucose level measurement. The woman will then drink glucose-containing liquid solution, and the doctor will test their blood glucose level every hour for 3 hours. The blood glucose readings will be compared to see how they measure up.
For the one-step OGTT, the following blood glucose levels suggest issues with absorption:
- On fasting: Above 92 mg/dL
- At 1 hour: Above 180 mg/dL
- At 2 hours: Above 153 mg/dL
For the second step of the two-step OGTT, the following blood glucose levels suggest issues with absorption:
- On fasting: Above 95 mg/dL
- At 1 hour: Above 180 mg/dL
- At 2 hours: Above 155 mg/dL
- At 3 hours: Above 140 mg/dL
If just one reading is higher than normal, the doctor may suggest a structured diet plan and a repeat of the test later in the pregnancy.
If two or more of the readings are abnormal, treatment for gestational diabetes will be prescribed and will include both a structured diet and regular physical exercise.
The doctor will likely request that a person monitor blood glucose levels to track the results of treatment during the course of pregnancy. People with abnormal glucose levels might need prescription medication if dietary adjustments and physical exercise are not enough to reduce and control blood glucose.
Risks and side effects
Most people do not show side effects after receiving the glucose tolerance test, and serious complications are very rare.
As it involves drinking oral material 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
Other diabetes tests
The glucose tolerance test is not the only way to diagnose diabetes. The following can also contribute to an accurate diagnosis and effective treatment:
- Hemoglobin A1C: This is a more common test that measures average blood glucose over 2 or 3 months. It measures the percentage of blood sugar attached to hemoglobin, the oxygen-carrying protein in the red blood cells. It is a quick and easy test.
- Fasting plasma glucose: This test measures blood glucose levels while a person is fasting. The person taking the test cannot eat or drink anything, except sips of water, for at least 8 hours beforehand.
- Random blood sugar test: Regardless of eating patterns, a physician takes a blood sample at any given time. People who have serious diabetes symptoms typically undergo random testing at this time.
There is currently no cure for diabetes. Early detection and immediate treatment are key to controlling the disease.
Glucose tolerance testing is an important diagnostic tool for identifying diabetes.
The testing doctor would consider any blood glucose reading above 140 mm/Hg to be possible or active diabetes. Two variations are available to test women who develop diabetes during pregnancy.
While there are minor risks relating to the oral material and injection element of the test, it is generally simple, safe, and accurate.
Take this test as early as possible to identify the risk of diabetes before it starts causing health problems.
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.