Blood work that your doctor may order act as preventive screens and diagnostic aids and may require you to alter your diet by fasting for part of a day or evening. Food and drinks other than water that you consume, including caffeinated beverages, can upset the precise measurement of your blood cells and the elements in blood plasma. Your doctor or staff at the laboratory that will take the blood sample will let you know whether, when and how to fast, depending on your scheduled tests.
Common Blood Work
As you age or develop risk factors for certain illnesses such as heart disease or diabetes, blood tests can give your health care provider positive or negative readings that confirm or deny potential diagnoses. Your sample may undergo single assessments of blood cholesterol, triglycerides, glucose or other nutrient levels, or several tests may be grouped, as in basic metabolic or lipoprotein panels. Depending on your health status, dietary substances that include caffeine may change your blood composition and skew the outcome or the interpretation of your test.
What Fasting Means
The strict definition of fasting is the suspension of eating, drinking or ingesting any substance other than water -- even beverages such as coffee and tea, in which water is infused with plant matter from steeping but not combined with it. This process still imparts organic elements, such as caffeine, that your body absorbs. While food can be withheld for several hours, water is needed to maintain a normal ratio of fluid and electrolytes in your blood. Fasting means that you add nothing to your digestive tract to change this balance.
Steps to Take
Take note of the time of day for which your blood draw is scheduled, so you can begin and end fasting at the correct time. Most fasts last for 9 to 12 hours. Your doctor will indicate the length of fasting appropriate to your specific blood test. Eat your last meal as you normally would within the given time frame, to provide adequate nutrition for routine body function until your next allowed mealtime. To help you stick with your fast, plan to make or buy coffee after you leave the lab.
Things to Avoid
Although drinking coffee may not affect some people's blood work, laboratories must maintain basic fasting standards for everyone in order to gain objective results. Avoid consuming anything other than water unless you are specifically instructed to do so. To safely satisfy your body's nutritional needs, don't fast for longer than 14 hours.
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