Can you do a different day? I try to do fasting blood work in the morning to mid morning. If not, you have two options. You can ask the last time you can eat to give you enough time for fasting blood work. Then eat something at that time. Sure, might be middle of the night. But it's an option. The other option is to just fast and see how it goes. You can't eat ANYTHING and only drink water for this blood work. But I've fasted for a fairly long period and you know what? It actually can feel kind of good. Just get through it. But see if you can change the appointment.