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Why do I have to fast before my blood test?

I?m having some blood drawn tomorrow for a blood test my doctor ordered. They said I can only have water and black coffee after midnight. Why do I have to do this?
Some routine blood tests require that the person not have any food for about 8 hours prior to getting the test. The two most common tests that require you to not eat are the blood sugar test and the blood cholesterol test. The blood sugar test requires that you don't eat because eating prior to the test will raise your blood sugar. If you have a large carbohydrate meal prior to the test your sugar will be high. This is normal. However, having a high blood sugar when you haven't eaten isn't normal. Fasting before this blood test will help your doctor interpret the results of the test. The cholesterol test is basically the same thing. Laboratory tests today are often sophisticated enough that the routine blood tests that used to require fasting don't always require it now. Sometimes doctors ask there patients routinely to fast prior to a blood test even if it is not required. They aren't doing this on purpose, but rather it is just a part of the blood work routine. If not eating breakfast is a big deal to you, then perhaps you should bring this up with your physician so that he or she can see if the testing can be done regardless of whether or not you ate. This is a good question that you can ask your primary care physician.
This answer is for general informational purposes only and is not a substitute for professional medical advice.
If you think you may have a medical emergency, call your doctor or (in the United States) 911 immediately. Always seek the advice of your doctor before starting or changing treatment. Medical professionals who provide responses to health-related questions are intended third party beneficiaries with certain rights under ZocDoc’s Terms of Service.

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