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How Reliable Is a Glucose Tolerance Test for Diabetes?

Helen Akers
By Helen Akers
Updated May 17, 2024
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A glucose tolerance test for diabetes is more effective when diagnosing gestational diabetes. The test can be effective in the diagnosis of type two diabetes, but may need to be administered several times. Some individuals who test outside of normal ranges may not develop diabetes if they modify their diet and physical fitness habits. In terms of long-term accuracy, the glucose tolerance test for diabetes is good at predicting the eventual development of the disease.

The purpose of a glucose tolerance test for diabetes is to measure how an individual's system responds to a high level of sugar. If the pancreas is unable to produce enough insulin to control the amount of blood sugar that circulates at one time, these levels can become higher than 140 milligrams per deciliter (mg/dl). Anything between 140 and 199 mg/dl indicates a glucose intolerance, which is considered to be a pre-diabetic condition. When a glucose tolerance test for diabetes yields results of 200 mg/dl or over, it can indicate that the individual has fully developed the disease.

In order to be as effective as possible, a glucose tolerance test for diabetes must be administered under certain conditions. The patient should fast for at least eight hours prior to the taking the test. Anyone who undergoes glucose tolerance testing needs to be aware that the results can be affected by other underlying diseases or conditions. Some doctors recommend consuming a high carbohydrate diet at least three days prior to taking the test.

When the test results indicate that a patient could have type two diabetes, a second or third test may be administered in order to confirm the diagnosis. This is because there are so many circumstances that might skew the test results, including physical activity level, mild illness, inability to fast properly, and timing. Some individual test results might indicate borderline blood glucose levels, which may need to be re-checked and evaluated for accuracy.

Glucose tolerance tests are not necessarily given to diagnose a case of type two diabetes. They tend to be used to determine whether a female has developed diabetes as the result of her pregnancy. The test is usually given to someone who has a good health history, is relatively physically active, and is not on any prescription medications that might skew the results. Drinking coffee and smoking cigarettes on the day of the test can also dramatically alter the test's accuracy.

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Discussion Comments

By anon1003140 — On May 05, 2020

A glucose tolerance test has a reliability of about 80-85%, i.e., repeating the test gives a different result in about 15-20% of cases. It also depends upon what you consume for three days before the test. You must consume some carbs (150gms) for at least three days before the test. If you consume less then you may show false insulin resistance. Some researchers think it is an obsolete test and should be discarded. It is not physiological - no one consumes 75gms of glucose in 10 mins in real life.

By donasmrs — On Feb 11, 2013

@turkay1-- I think it depends on the doctor. I didn't have to go for a second test, I think my doctor trusts the lab at my hospital a lot. Some doctors might ask for a confirmation test.

What happens is that you have to go to the hospital in the morning without eating or drinking anything. They will take your blood and then they give a glucose sugar solution for you to drink. One hour after drinking the solution, they take your blood again. One hour later, another blood sample is taken.

After that you can eat, you just wait for the glucose tolerance test results. You might also have to give blood for an insulin test to see how much insulin your body produces.

By candyquilt — On Feb 11, 2013

Can anyone tell me what is done exactly during a fasting glucose tolerance test? If the test shows that I have diabetes, do I have to go for a second test later?

By burcinc — On Feb 10, 2013

A glucose tolerance test is reliable. However, many people forget that there are different types of diabetes. Some people's blood sugar levels are too high after two hours of eating, others are too high after one hour. Both of these are considered type two diabetes. The difference is that if the doctor asks for a 2 hour glucose tolerance test, where only blood sugar levels at the second hour are checked, diabetes might go undiagnosed.

I have type two diabetes and I did a lot of research about the topic before I asked my doctor for a glucose tolerance test. I had all the symptoms and I didn't want to be misdiagnosed so I specifically asked for a 3 hour glucose tolerance test.

My blood sugar levels at starvation and after two hours were normal. But my one-hour post-meal blood sugar level was 240mg/dl which is too high. This is how I was diagnosed.

If anyone suspects diabetes, please ask for a 3 hour glucose tolerance test where blood sugar will be tested at every one hour interval.

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