alt Hacker NewsI've not seen evidence that creatine actually has significant impact on eGFR. Anecdotally, mine does not budge even on 5g a day. Meta-analysis show minimal impact, e.g. https://pmc.ncbi.nlm.nih.gov/articles/PMC12590749/
Muscle mass obviously does, though. cystatin c is a better market if your body composition differs from the "average"
I did end up taking a cystatin c test privately to be able to prove to my GP that the results he freaked out over were nonsense. I'm in the UK, and for whatever reason the NHS just doesn't typically do them for basic kidney function - presumably cost, but they were dirt cheap to do privately so...