Rules on How to Fast for Blood Work

By Alicia Bodine ; Updated July 27, 2017

Blood tests that check sugar or cholesterol levels must be performed only after a patient has fasted. Fasting allows a doctor to get an accurate blood result in order to make a diagnosis. If a patient doesn't follow fasting rules before a blood test, he will be asked to reschedule.

Make an appointment with your blood laboratory. Schedule a morning appointment if it is difficult for you to go without food for long periods.

Determine the time to begin your fast. Count back 12 hours from the appointment time. For example, if your appointment is 7 a.m., 12 hours earlier is 7 p.m. This is the time you must begin fasting for your blood test.

Drink only water after you begin the fast.

Consult your physician if you are taking medication.

Abstain from smoking, sucking on mints or chewing gum during your fast. You may resume these activities once your blood has been drawn.


Do not drink coffee until you complete the test.

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