Patients often need to fast before having blood drawn, because their test results may not be accurate otherwise, cautions Cleveland Clinic. This means that a person should not eat or drink for 10 to 12 hours before their appointment. This includes black coffee, which will exacerbate fluid loss due to increased urination. To make things easier, consider scheduling your procedure during morning hours so the fasting takes care of itself as you sleep. The exception to this rule, of course, is usually water.
You’ll want to start zeroing in on your water intake the day before getting blood drawn. Phlebotomy Training Specialists suggest sticking to 64 ounces in a day, topping that off with a 16-ounce glass of water right before the appointment. In some cases, however, even water may not be permitted beforehand, so be sure to consult with your phlebotomist so you can properly prepare.
While pre-blood work hydration is certainly important, so is hydration after the fact. Therefore, don’t neglect your fluid intake as soon as your appointment is over.