Highly probably both and neither. Unless you have done the MRI and oxygen scan, the value they are quoting from blood tests as a kidney function % will be the eGFR. This is the ESTIMATED glomerular flow rate (I.e. fluid through kidneys) and is well known to be highly variable, and somewhat inaccurate. One result is pretty useless by itself, but an increasing or decreasing trend over several results is likely to be real.
For example, my GFR (no 'e') as of a month ago is 67.8. Over the last six months my eGFR has been between 30 and 60 so not only wrong but consistently low.