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Fasting Blood Tests & Black Coffee

by
author image Nancy Clarke
Nancy Clarke began writing in 1988 after achieving her Bachelor of Arts in English and has edited books on medicine, diet, senior care and other health topics. Her related affiliations include work for the American Medical Association and Oregon Health Plan.
Fasting Blood Tests & Black Coffee
A large cup of black coffee. Photo Credit Mindstyle/iStock/Getty Images

Caffeine addicts must pay special attention to their doctors' orders for blood tests needed for routine health checks and diagnostics. Depending on the type of blood work, typical laboratory protocols can upset your coffee-shop routine with temporary dietary restrictions. Your doctor may request a blood specimen that requires previous fasting for accurate analysis. In most cases, you will have to suspend your indulgence in food, coffee, tea, energy drinks and other beverages for several hours in order to produce a viable blood sample.

Fasting Tests

Common fasting tests evaluate blood levels of glucose, triglycerides, HDL cholesterol and LDL cholesterol, and C-reactive protein to gauge diabetes and cardiovascular risks and conditions. If your health care provider adds non-fasting tests such as red blood cell counts or kidney function tests to a fasting blood profile, you will still have to fast. This means going without food or drink other than water for a certain period of time.

Black Coffee Effects

You may think that a water-brewed drink such as black coffee won’t upset your fast, but the caffeine and other organic compounds in coffee beans that infuse the water will. These substances can change your blood chemistry or alter body functions to do so and therefore threaten the objectivity of your blood sample analysis. The Food and Drug Administration reports that caffeine remains in your system for up to six hours, causing your stomach to secrete larger amounts of acid and your kidneys to excrete more water. This causes a pH and body fluid imbalance that may affect the outcome of your blood test. Even decaffeinated coffee contains some caffeine and other compounds that can taint your blood sample.

Fasting Instructions

A water-only fast involves not eating or drinking anything but water, generally for eight to 12 hours prior to your blood draw. Your doctor will let you know if and when you should be fasting or will refer you to the testing laboratory for explicit instructions. Unless you are given an exception, you should not eat any food during this time or drink hot or iced black coffee or tea, soda, juice, or any liquid but plain water.

Tips for Success

Note your scheduled day or time for you blood test and choose a start time for your fast, based on your doctor’s instructions. Eat a nutritious dinner as usual before the cutoff time to ensure adequate blood glucose for energy while you fast. If caffeine dependency threatens your willpower, prepare a Thermos of coffee the night before your test or plan to visit a nearby cafe after your appointment.

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