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Certain laboratory tests prescribed by your doctor require you to fast for a period of time prior to having your blood drawn. Fasting prevents food and drink from interfering with the test results, which can make them inaccurate. Tests that require fasting might include cholesterol panels, lipid panels, metabolic tests and HDL or tryglyceride level panels.

Fasting Defined

Fasting means you do not eat or drink for a specified period of time. Typically you are allowed to drink only water during the fasting period. Tea and coffee contains caffeine, which can make your test results inaccurate. For fasting labs you might also be asked not to chew gum, smoke or even exercise. Exercise, like digestion, can effect your test results.


Your doctor should tell you how long you are required to fast prior to your test. Most fasting lab tests require 8, 12 or 14 hours of fasting. If you are not certain how long you are supposed to fast, call your doctor's office or the lab to ask. If you take your tests and have not fasted the proper amount of time you might have to repeat the tests.

Scheduling Your Test

The easiest time to fast is while you are sleeping. If you can schedule your appointment for the morning you can start your fast after dinner the night before. This makes the fasting period seem easier to get through as you are not trying to work, take care of your family or perform any other activities without any food or drink to keep you energized. If you cannot schedule your appointment for the morning then limit your scheduled activities for the day as fasting can make you feel both tired and weak.


If you take any medications, especially medications requiring you to take them with food, consult your doctor about what to do during your fasting period. You might need to skip a dose of your medications while fasting, or your doctor might feel it is fine for you to continue on your normal medication schedule.