As part of a routine physical examination, your health care provider may prescribe blood work to assess your physical condition. Checking your blood for accurate nutrient levels and blood cell counts may sometimes require you to fast for a brief period, usually overnight, before a practitioner draws your blood in the laboratory. You must avoid anything that might change the outcome of your test – even a cup of coffee.
Fasting Blood Work
Other than water, food and drinks that you consume can add to or alter components in your blood that affect your sample and your doctor's interpretation of your health. Blood tests that need a dietary clean slate can include individual samples to measure blood glucose, cholesterol or triglyceride levels or multiple screens such as lipid or basic metabolic panels.
You might also need to fast if your doctor merges nonfasting tests such as complete blood counts and checks for enzymes, vitamin D or other nutrient levels with tests that do require fasting. Some blood work, such as glucose tolerance tests, is performed on two or more separate occasions to ensure accuracy. The investigating laboratory may instruct you not to eat after dinner the night before and to hold off on your morning breakfast, coffee or tea until after the blood draw.
Fasting is defined as eating and drinking nothing but water. These narrow parameters allow lab technicians and physicians the greatest diagnostic margin by introducing no dietary variables into the procedure. While you may consider black coffee little more than water, drinking it causes you to absorb caffeine and other organic compounds into your bloodstream. You need only plain water, without added vitamins, flavoring or carbonation, to achieve the correct fluid and electrolyte balance in your blood chemistry. In some instances, your doctor will ask you to avoid only certain foods, beverages or medications before a test.
How to Prepare
Your doctor will inform you what time your blood test is scheduled, whether to fast and, if so, how long you must fast beforehand, generally between eight and 12 hours. Prepare for a water-only fast by eating a nutritious meal prior to the fasting period. Help yourself adhere to instructions by chilling some water in the refrigerator and planning how you will break your fast after the blood test. You might pack a breakfast and even an insulated bottle of coffee before beginning to fast.
How to Fast
To fast effectively and provide the best possible blood sample, simply follow the instructions provided by your doctor or the testing laboratory to the letter. Check the time of your appointment and count backward to the appropriate start time. In a water-only fast, do not consume any solid or liquid food and do not drink any coffee, soda or other beverages. End the fast when directed by eating another nutritious meal with your beverage of choice.