if you are running a glucose test then you shouldn't as that will effect that reading.
also creatine can effect your BUN levels and creatinine levels but that sticks around a long time in the body so keep that in mind.
if you are running a full panel and not concerned with your glucose numbers then just eat who cares. it won't effect your other numbers, if anything going into a test de-hydrated and hungry can actually spke your kidney numbers and lower your test
the lady at the lab is gonna ask you if you run a full panel "have you eaten"? just lie to her. not her business how you run your test anyway .. hell i even use an anonymous name and if they ask for your ID just walk out.