A patient fasting for blood tests may have water during his fast, according to Quest Diagnostics. It is advisable to drink a lot of water during the fast because it is harder for care providers to draw a patient's blood if the patient is dehydrated.
A patient must generally abstain from food and liquids, except for water, for an established period during a fast, says Quest Diagnostics. The period of fasting usually lasts from several hours to overnight. The purpose of fasting before blood is drawn is to eliminate the nutrients that pass into the bloodstream from food. These nutrients can potentially interfere with accurate test results. A patient must commonly fast before blood tests that measure glucose or cholesterol in the bloodstream.
A patient who takes daily medications should continue with the prescribed routine during a fast unless his physician prohibits them, suggests Quest Diagnostics. A patient should check with his doctor about taking vitamins and other supplements during the fast. After the test, the patient may break the fast by having a snack or meal. Morning is often a good time to schedule a fasting blood draw since much of the fasting takes place while the patient is asleep. Patients who think that fasting may be a hardship should consult their physicians.