When your doctor orders a fasting blood test, you will be instructed to fast for a specific period before having your blood drawn at the laboratory. Fasting blood work requires that you not eat or drink anything other than water for the specified preparatory period. You cannot drink coffee before a fasting blood test because it may skew the results.
Coffee Counts
Many people mistakenly believe coffee contains no nutrients. Although the calories and nutrients in coffee are present in low concentrations, they are substantial enough to cause fluctuations in certain substances in your bloodstream. For example, your total cholesterol and LDL, or "bad" cholesterol, levels may increase after drinking unfiltered coffee.
Fasting and Reference Ranges
Your doctor interprets your blood test results by comparing them to standard reference ranges. Clinical laboratories establish reference ranges by testing the blood of hundreds to thousands of apparently healthy volunteers under specific circumstances and determining the range of values most people in the population exhibit. For fasting blood tests, all of the volunteers fast prior to their blood draw. If you have broken your fast by drinking coffee before a fasting blood test, the established laboratory reference range no longer applies and your doctor may have difficulty interpreting your test result.
Blood Tests Requiring Fasting
Although fasting is not required for most blood tests, it is important to comply with this requirement if your doctor orders a fasting blood test. Examples of blood tests that typically require you to refrain from eating or drinking before testing include fasting glucose, glucose tolerance test, total cholesterol, HDL cholesterol, LDL cholesterol, VLDL cholesterol, lipid panel or profile, gastrin, insulin, lipoprotein subfractions, and triglycerides. If you are not sure whether you need to fast for your blood test, call your doctor or the testing laboratory. Also be sure you know how long you need to fast before your test.
I Forgot!
It happens sometimes; you have good intentions but forget about your fasting blood test and drink some coffee or eat something the morning of your test. If this occurs, call your doctor's office to determine whether you should reschedule your test. Depending on what tests your doctor ordered and your current medical problems, your doctor will decide whether you should go ahead with your scheduled blood work or reschedule.
To prevent making this mistake, place a sticky note on your coffee maker, cupboard door or some other prominent place to remind yourself not to eat or drink anything the morning of your blood test.
References
- USDA Agricultural Research Service: What's in the Foods You Eat Search Tool
- "American Journal of Epidemiology"; Coffee Consumption and Serum Lipids: A Meta-Analysis of Randomized Controlled Clinical Trials; Sun Ha Jee, Ph.D., M.P.H., et al.; 2001
- Lab Tests Online: Reference Ranges and What They Mean
