You are in a tough position. Your reputation has a lot to do with perception of guilt here. Info from other patients of yours may help, the family of the patient in question or perhaps even the patient. I assume this has come from a fellow employee. If it has come from other than a fellow employee including the patient's family then you may use the testimony of co-workers to help you. A good past work record will help you but if there are issues there then this person's allegations may stick.