It doesn't help when doctors are often unaware of outliers affecting the test results. E.g. I've had a number of doctors freak out over my eGFR (kidney function) test results because the default test they use is affected by body mass and diet, and made even worse by e.g. preworkout supplements with creatine. None of my doctors have been aware of this, and I've had to explain it to them.
I'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"