Getting ready for a blood test may involve abstaining from coffee and other breakfast favorites if your doctor tells you to fast. A comprehensive test such as a metabolic panel uses one blood sample for a variety of assays -- some of which require fasting. To accommodate lab specifications for all of the tests, which include measurements of your blood glucose, minerals, proteins and other elements, your sample must be drawn after a period of consuming water only.
Metabolic Panel Requirements
Your doctor may order a basic or comprehensive metabolic panel of blood tests for preventive screening or when you have health symptoms that require diagnosis. This blood work checks your liver and kidney function and may require several periodic tests for comparison. Even if you’ve had a metabolic panel before, follow your doctor’s instructions for fasting, which may have changed. This will ensure that your blood sample reveals the current status of your electrolyte, blood sugar and acid/base balance as well as other aspects of your blood chemistry.
Coffee Affects Blood
Despite its water ingredient, brewed coffee and other caffeinated beverages actually lower your level of body fluids through their dehydrating action. The compounds in coffee beans make your stomach secrete extra digestive acids, which alter your body’s pH level. If you add milk or sugar to your coffee, your blood glucose level will increase. While caffeine dissipates from the bloodstream in as little as four hours, it can much longer for your kidneys to readjust your body’s electrolyte and acid/base balance. Any of these consequences can affect your blood sample.
When to Fast
Most laboratory protocols request a fast of between 8 and 12 hours for basic or comprehensive metabolic panel. Your doctor will specify the length of your fast, which will begin the night before your appointment. Arrange for a morning blood draw and calculate what time to stop eating and drinking coffee and other beverages the night before. Adhere to the water-only rule after a regular dinner, and plan to postpone your morning coffee until after you provide a blood sample.
When to Drink Coffee
You may be craving caffeine by the time you successfully end your fast, but your blood glucose level will be low after going without food overnight. To avoid symptoms of hypoglycemia such as light-headedness, drink your delayed cup of coffee with a nutritious breakfast.
- American Association for Clinical Chemistry; Comprehensive Metabolic Panel; March 2009
- Food and Drug Administration; Medicines in My Home, Caffeine and Your Body; Fall 2007
- Merck Manual Home Health Handbook; Overview of Acid-Base Balance; July 2008
- National Heart Lung and Blood Institute; Common Blood Tests; January 2010
- MedlinePlus; Hypoglycemia; July 2011