Consider not exercising for at least 2 days before a creatinine blood test because the proteins will be affected by muscle metabolism. Stay hydrated but not too much salt(s) as well. I'm 71 and used to avg 160 mi/week on my bikes, but diagnosed with stage 3a CKD last summer so ramping down (no known risks other than having covid)

GFR uses only creatinine because it's the one that is least effected by what you eat so it's considered the most reliable. That's what I was told anyway. I was also made aware that many consider Cystatin as a more relible gage than GFR