[dragonwriter]

I would suspect that there is a selection bias here, as a whole lot of the low end of the general health distribution would prevent you from donating a kidney, or at least make it less likely. So you'd expect them to do better than average if donation had no effect.

To know if there is an effect, you need to not compare to the general public, but to a subset similar in factors which are associated with the outcome measures to kidney donors.