Are your eGFR kidney filtration blood tests accurate if you lift weights, take creatine or hold a large amount of muscle?
If you're doing DIY blood work, Cystatin-C is probably the test you want. eGFR is called "e" GFR for a reason - it's an estimate based on a simple algorithm considering age, weight, and creatinine blood levels.
If you're sedentary, that's great. If you train with weights and have higher than normal muscle mass and / or take creatine supplements, then likely your eGFR will be underestimated, because the equation involving creatinine fails to consider those with high BMI (muscular BMI, not fat people).