Question: how do you have to fast before a blood test?
Answer: Fasting means you don't eat or drink anything but water usually for 8 to 12 hours beforehand. So, if your appointment is at 8 a.m. and you're told to fast for 8 hours, only water is okay after midnight. If it's a 12-hour fast, avoid food and drink after 8 p.m. the night before.