There are some blood tests that are extremely sensitive to food or drink. It is necessary for a patient to fast for several hours prior to having one of these tests drawn. It is important to find out if fasting is required for the type of test being performed.
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Tests that Require Fasting
Blood cholesterol, triglyceride and glucose levels are the most common tests that require fasting. Vitamin levels, such as vitamin E or A, also need to be drawn when the patient is fasting.
Typically at least a 12-hour fast is required for cholesterol and triglyceride screenings. Vitamin levels and glucose tests usually only require an eight-hour fast. Following the guidelines is important to make sure your blood sample is not influenced by food or drink.
What is Allowed?
Water is allowed even when fasting. Usually your regular dose of medication can be taken, although some medicines do affect test results, so check with your physician first. Black coffee or tea may be allowed, but again, check with the laboratory or your physician, because caffeine can alter some test results.
What is Not Allowed?
No food at all is allowed. Gum chewing, especially of gum with sugar in it, is forbidden. No liquid, other than water, should be consumed without the express permission of your physician.
Results may be inaccurate if fasting was not done prior to the test. This might lead to treatment for a disease such as diabetes or a condition such as high cholesterol that you do not have. The specimen will need to be repeated because an accurate result is necessary.
Keep in Mind
If your physician informs you of abnormal test results, ask if fasting was required. If you did not fast as needed, tell your physician immediately.