So, your healthcare provider has asked you to fast before your blood test. Fasting means not eating and drinking for a period of time. You often need to do this before bloodwork. This is because what you eat and drink ends up in your bloodstream and can skew your results. Show
Advertising Policy Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy But can you drink black coffee when fasting for bloodwork? How about taking your medications? Family medicine specialist Timothy Tramontana, MD, MS, answers your frequently asked questions. Q: How long should you fast for a blood test?A: Typically, you should fast before bloodwork anywhere between 10 and 12 hours. Usually, people choose to do blood tests first thing in the morning. This way, you can fast overnight, when you generally don’t eat anyway. Types of fasted blood testsBlood tests are a quick, effective way for doctors to check your health and to see if certain treatments are working. Blood tests you may need to fast for include: Advertising Policy
Q: Can you drink water before a fasting blood test?A: Yes. You can drink as much water as you want, but it should be plain. Avoid lemon-flavored and carbonated waters, as well as teas until after your 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. Many people think coffee counts toward your fluid requirements. However, it makes you urinate more, which can affect some of your lab results. So, get your blood test done, then get your cup of coffee! Q: Can you take medication before a fasting blood test?A: You can take over-the-counter and prescription medications unless your doctor advises you otherwise. On that note, it’s definitely a good idea to tell your doctor about all medications and supplements you’re taking to avoid something potentially skewing lab results. Advertising Policy There’s no indication that daily medications — such as blood thinners and blood pressure meds — will cause any problems with fasting bloodwork, so definitely continue taking them unless told otherwise. And if you have a headache, taking an ibuprofen or another over-the-counter pain reliever shouldn’t cause any issues. Q: What if you forgot to fast or ate something by accident?A: Let your doctor know so you don’t end up doing a test unnecessarily. If you’ve already gone for the test without fasting, and the results come back normal, then you’re most likely OK. But if results come back abnormal, that may mean you have to redo the bloodwork while fasting. Q: What if you have more questions about your test?A: Call or send a message to your primary care team. Someone should be able to field your questions so you can feel as prepared as possible. 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. 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. How important is it to fast for 12 hours before blood work?Fasting before certain blood tests is important to help make sure that your test results are accurate. The vitamins, minerals, fats, carbohydrates, and proteins that make up all food and beverages can impact blood-level readings, clouding the results of your test.
Is 7 hours of fasting enough for blood test?How long should you fast before a blood test? The amount of time you need to fast for will vary depending on the test. For most tests, you will be told not to consume anything but water for eight hours leading up to the test. For a few tests, a 12-hour fast may be needed.
Can I drink water before a fasting blood test?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.
What is the fasting rule for blood work?For blood tests, do not eat or drink for 8+ hours before visiting the laboratory. For lipid tests, do not eat or drink for 12 hours before visiting the laboratory. It is okay to drink water, but not coffee, juice or tea. Do not smoke during the fasting period.
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