It depends on the type of blood test you're having. The healthcare professional arranging your test will tell you if you need to do anything to prepare for it. Show
You can eat and drink as normal before some blood tests. But if you're having a "fasting blood test", you will be told not to eat or drink anything (other than water) beforehand. You may also be told not to smoke before your test. Common fasting blood testsExamples of blood tests that may require you to fast include:
For more information about a wider range of blood tests, go to Lab Tests Online. Further information
Page last reviewed: 25 February 2021 As a service to our readers, Harvard Health Publishing provides access to our library of archived content. Please note the date of last review or update on all articles. No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician. Certain blood tests require fasting beforehand. If your healthcare provider has instructed you to fast before your upcoming test, it means that you should not eat or drink anything (besides water) for the specified amount of time before having your blood drawn.Understanding why fasting is important and how to properly fast before a blood test can be helpful for eliminating pre-test anxiety and simplifying the testing process. Below, learn how to fast for a blood test correctly to help make sure that your test results are accurate. Why Is Fasting Required Before Blood Tests?Fasting before a blood test helps improve the accuracy and reliability of the test. Accurate test results are a critical component of diagnosing various health conditions and monitoring the effectiveness of treatment plans. Your diet contains a range of nutrients, including carbohydrates, proteins, fats, minerals, and vitamins. When you eat and drink normally, those nutrients are broken down and absorbed into your bloodstream. This can impact the levels of those substances in the blood and skew lab test results. For example, certain methods of diabetes testing measure the patient’s baseline blood sugar levels after a period of fasting. Eating directly before the test will cause the patient’s blood sugar to spike, leading to an inaccurate test result. The patient could be misdiagnosed with prediabetes or diabetes unless their doctor recommends that they complete a second test. Which Blood Tests Require Fasting?Not all lab tests require fasting beforehand, but many of the most common blood tests do. Tests that typically require fasting include:
Can I Drink Water Before a Blood Test?Yes, you can drink water while fasting before a blood test—in fact, drinking plenty of water can help ensure that you receive accurate test results. Dehydration can affect certain blood tests such as cholesterol, electrolyte, and BUN tests. Be sure to stay hydrated before your test by drinking the recommended amount of water for your weight and activity level. Can I Drink Coffee / Alcohol / Juice / Soda / Tea While Fasting?No. Any beverage other than water should not be consumed before a blood test unless approved by your healthcare provider. Alcohol, soda, and juice contain high quantities of sugar, which can interfere with many common test results. Black coffee, tea, and other caffeinated beverages are diuretics, which can have a dehydrating effect and cause test results to be inaccurate. For best results, avoid drinking all non-water beverages for the recommended amount of time before your test. What If I Accidentally Eat or Drink Before My Blood Test?If you have been instructed to fast before your blood test and you accidentally eat or drink something during the fasting window, it is important to notify your healthcare provider or the person drawing your blood. If you don’t, your results could be incorrectly interpreted. It may be possible for your healthcare provider to interpret the non-fasted test results. Most likely, they will instruct you to reschedule the blood test at a time when you will be able to successfully complete the fast before the test. Your physician will let you know the steps you need to take before your blood test, including whether or not fasting is required and for how long. By following all testing instructions and best practices (including how to fast), you can ensure a smooth testing process and the most accurate results. Are you a patient in need of lab testing? Find a convenient testing location and schedule an appointment near you. Can I have coffee before a fasting blood test?Can you drink coffee if you're fasting before a blood test? Even if you drink it black, coffee can interfere with blood test results. That's because it contains caffeine and soluble plant matter, which might skew your test results. Coffee is also a diuretic, which means that it will increase how much you pee.
Can you drink black coffee while fasting for a blood test?Q: Can you drink black coffee when fasting for bloodwork? A: Don't drink anything but water. Coffee contains caffeine. It's also a diuretic, which removes water from your body.
How long before a blood test can you drink coffee?A bite of toast and a few gulps of coffee won't really make a difference, right? Not so fast. Your results could come back wrong if you give in to temptation. Fasting means you don't eat or drink anything but water usually for 8 to 12 hours beforehand.
Will one cup of coffee affect blood test?For example, a 2005 study found that drinking one cup of coffee within an hour of a cholesterol test does not significantly impact the clinical test results. Specifically, black coffee did not significantly affect TC/HDL-C levels, and coffee with sugar and creamer did not significantly alter triglyceride levels.
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