A lot of people think you can't eat or drink before having bloodwork drawn. You can, it just depends on what tests your doctor ordered.

If your doctor ordered any panel that requires measurement of glucose (such as in a BMP or CMP) or triglycerides (such as in a lipid profile) then yeah, you shouldn't eat if you want an accurate result. Otherwise it doesn't matter

There are a few specific tests in addition to glucose and triglycerides where eating may affect the result but most people aren't getting those measured so i'm not even gonna mention it