Your doctor may have prescribed a fasting blood test to reveal your health status or risk factors for disease. If you're not used to fasting, this seemingly simple requirement can raise many questions about what you can and can't eat, drink or ingest. Anything that you take in, including vitamins or other supplements, may alter your blood chemistry to a degree that affects its analysis. In some cases, your doctor may tell you to skip your morning vitamin and wait to take it until after your blood draw.
Any of a dozen or more blood tests may entail prior fasting, including those that measure blood glucose, lipoprotein, cholesterol, enzyme, vitamin and other nutrient levels. Because tests may be ordered for different reasons and health conditions vary from patient to patient, fasting directions are not the same for every person or every type of blood test. Depending on their impact on your particular test, substances such as food, beverages, medications and vitamin, mineral or herbal supplements, may be denied for a period prior to your scheduled appointment. You should discuss your current diet with your doctor, as well as all medications you take, and receive or request detailed instructions beforehand.
Your Current Diet
Consider that compounds from everything you eat, drink, chew, swallow or smoke on a regular basis, enter your bloodstream. Any of these elements can affect the outcome of your blood test and necessitate another sample or even cause a misdiagnosis. The particular test or group of tests that you are having will determine whether you can have any, some or none of these substances during the fasting period.
Your Fasting Diet
Your health care provider or laboratory staff will let you know exactly what you can and cannot have while fasting according to test protocol. Some tests require a water-only fast. Some may or may not allow vitamins and supplements, black coffee or tea, or regular meals with certain exclusions. If you're being tested for vitamin A, for instance, you may be instructed to suspend your multivitamin dose. Factors in your individual instructions include your current health condition, dietary habits and the goal of the blood work. Even if you've had the same test before, fasting directions may change.
Fasting for blood glucose tests may last as little as two hours, while most other tests need eight to 12 hours of dietary abstinence. Eat a balanced meal prior to your fasting period and again after your blood sample has been taken. Meet every criteria your doctor gives you, and if you make a mistake and take one of your supplements, let the laboratory technician know as soon as you arrive for your blood test.