A lot of people here know what a nephrologist is, so I would use that term. If they look confused, I would explain or say "Kidney doctor".Thanks a lot, YLP. I think native speakers tend to use the language in an easier way, so they avoid complext words. May I ask if you would say "She is a nephrologist" if you have a friend who's a kidney doctor, I don't think so. Nor do I think that AE or BE speakers would say so.