I've even had doctors unaware that the regular eGFR tests needs to be adjusted for body mass to be remotely accurate if you're bigger than average - e.g. lifting weights or obese. Couple that with creatine and you get some "fun" moments when your doctor tries to gently break to you how messed up they think your kidneys are. The first time I had that I'd warned my doctor ahead of time to expect elevated creatinine, and he still freaked out.