Your GFR is computed using serum creatinine levels. So if you lift weights or have a lot of muscle mass, the test will be greatly skewed. Take 3 days off completely from the gym and retest. If you are taking a creatine supplement, this can also skew the creatinine reading. If your kidneys were an issue you would have a host of other items wrong on your blood test. Low albumin, high triglycerides, lowered hdl, high urea(BUN), etc. As crazy as it sounds, the best supplement for kidney health is milk thistle. Google Silymarin, which is milk thistle, it does work and has studies to prove it.