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February 6, 2006

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Question from Melbourne, Australia:

I am to have a fasting blood test for diabetes. I've read a previous question on this site regarding someone who drank large quantities of orange juice prior to testing who believed that this may have affected the result. The reply said that if undergoing a oral Glucose Tolerance Test (GTT) that it is very important to eat 100 grams of carbohydrate for several days prior to the test to avoid a false postive test. Do these carbohydrate guidelines also apply to a normal fasting blood test? Are there any other eating guidelines that I should follow prior to testing? Also, do vitamin C and citrus affect the result?

Answer:

As long as you have fasted overnight to make it a fasting glucose sample, you should be okay. The oral glucose challenge test uses an oral glucose load and measures glucose levels after the load. It has been shown that those who withhold glucose may have false positives (elevations in glucose), if they have not had recent exposure to carbohydrates prior to the test. It is less critical for the fasting glucose.

JTL