What Factors Affect Glucose Tolerance Test Values?

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  • Written By: Erin J. Hill
  • Edited By: Bronwyn Harris
  • Last Modified Date: 17 October 2019
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Various factors may have an impact on glucose tolerance test values, including food consumption, exercise, and certain medications. Patients should follow all doctor instructions before and during testing to ensure accurate results. Most times, patients are only allowed to consume water during the period of testing and for several hours beforehand as well.

A glucose tolerance test can last anywhere from one to four or more hours, depending on initial results. Patients who are being tested for either gestational or type 2 diabetes are asked to refrain from eating for several hours prior to testing. Upon arrival at the doctor's office or clinic, patients drink a water solution containing glucose. They are then tested one hour later to check for high blood sugar. If this test shows spiked blood glucose levels or is inconclusive, additional tests may be conducted in which blood sugar is tested one, two, and three or more hours after glucose consumption.

Many factors may affect glucose tolerance test values. The first, and one of the most common, is consuming food or drink during the testing period. This can artificially raise blood sugar higher than normal and may lead to a false diagnosis of diabetes. For this reason, patients should make sure they are very clear on when to stop eating and drinking anything other than plain water and for how long this process should continue.


Exercise directly before or during testing may also impact glucose tolerance test values. This is because working out alerts the body that more glucose is needed in order to function, resulting in faster glucose consumption and lower overall blood sugar. Those who are diabetic may experience increased blood sugar when exercising vigorously because the body releases glucose faster than it can be used. Both of these things may skew test results in one direction or the other.

There are also certain medications which may affect glucose tolerance test values. Any pills or drugs containing any form of sugar may raise blood glucose levels. Medications which affect insulin production or the metabolism may also impact blood sugar by causing the body to burn sugar faster or slower than usual. Patients who are unsure of whether a medication they are taking may impact test results should discuss it with a doctor prior to testing. In some cases, the drug may be temporarily discontinued or lowered to allow for more accurate results.


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Post 3

@ysmina-- That's right, you're not supposed to eat for at least eight hours before the fasting glucose tolerance test.

Also, if anyone takes beta-blockers (high blood pressure medication), estrogen, contraceptives or anti-psychotic drugs, make sure to tell your doctor. These drugs can cause insulin intolerance and may show higher glucose levels than normal. Your doctor will work out something and may ask you to stop taking the medications for a short time if that's possible.

Post 2

@ysmina-- Yea, I had a glucose tolerance like that once. The rules are the same, you're not supposed to eat before the first blood draw. You can drink water and don't exercise again. Also, I was confused about taking my diabetes medication after I ate my breakfast for the second blood draw. I asked my doctor and she told me to take my medication. You might want to ask your doctor about that to make sure.

Also, after you have had your breakfast and start waiting for an hour for the second blood draw, don't keep eating. Finish your meal and then start counting the hour and don't have anything other than water in that time period. Otherwise, your glucose levels will keep increasing and the test won't be accurate. It's common sense but I've heard that some people keep eating in that time frame.

Post 1

When I had my first glucose tolerance test, of course I didn't eat anything for my fasting glucose blood draw. I didn't know if drinking water would affect the results, so I didn't drink water either.

The test I had is called a "3 hour glucose tolerance test". I gave blood, then I drank the glucose mixture. I waited for an hour, gave blood again, waited for another hour and gave blood for the last time. And I was at the hospital the whole time, so I didn't exercise or anything like that. I think my results were very accurate for this reason.

I'm supposed to have another tolerance test pretty soon without the glucose mixture. The doctor

said that I would give blood three times again, but instead of having the glucose, I will have my breakfast.

I'm not sure how this is going to work out and if there are any additional rules with this test. Does anyone know?

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