Such a good question. The best thing is to ask the person in question what they would like to be called. AT my Mum's nursing home, which was attached to a hospital, all staff had name tags with their designation and their name. For the Doctors it was their last name, after the Dr. For most others it was their first name followed by RN, RNA, PA, PSW and others. Some had their last name on too. And of course much of the time you couldn't see the name tag well, especially Mum who was mostly looking up at them; plus there were too many to remember. We, including Mum and my sister who was an RN in the hospital, mostly called them Nurse if we needed them for something and had to call them over. Mum called all the men Doctor and all the women Nurse. There was no way to tell for sure what the proper designation was and nobody got snotty about it.