I wouldnt live and die by that calculator. Amputees and bodybuilders will always have a higher GFR than what is shown on bloodwork....but that can also give you a false feeling of security.
When I saw a person referred to on another thread with the statement "my creatinine was fine it was 1.5 and then this last time it was 2.0" I looked at the screen in disbelief. You are on a road of trouble if your creatinine is plus 1.5 consistently. 2.0 you are in really big trouble. Ive been training for 35 years, I have never ever had a creatinine level outside the average markers, not even close. If your creatinine is high, your gfr is low, go get a cystatin C and if that is out of range....its time to buckle down and think about more important things than this bodybuilding stuff. I tell people all the time you PM me on instagram with kidney problems "I want you to really research dialysis and all that it entails....you will absolutely hate your life going through that on a weekly basis....the freedom you have right now will be taken away from you"