How many hours before a blood test should a person fast?

Depending on the type of blood test being performed, a patient is often asked to fast for eight to 12 hours, according to the National Heart, Lung and Blood Institute. Fasting involves the abstinence of all food or drink for a predefined time period to ensure accurate testing results.

Fasting is often necessary because consumed foods and drinks eventually make their way into the blood stream, where they can affect the levels of chemicals measured by blood tests, states Quest Diagnostics. However, in the case of cholesterol testing, researchers say that fasting does not affect the results, reports the New York Times.