Leftseat.com shows 37 kidney transplants holding 1st class medicals.

https://www.leftseat.com/sistats.htm

FAA Guide for Medical Examiners:


https://www.faa.gov/about/office_org...se_prot/renal/

Old, and military-related, but on point:

One Kidney - Aviation Medicine - Baseops Forums

http://www.asams.org/guidelines/Comp...%20Disease.htm

Bottom line: decided on a case by case basis.