Thanks for the query.
GFR is calculated by a formula which uses serum creatinine
as one of the parameters. The other parameters are age, sex, race, body weight.
Thus it is quite unusual for GFR to decrease if the serum creatinine is the same, unless you have lost some weight.
The reason for GFR decline can be natural progression of the disease or some new disease process. Your nephrologist
is the best person to evaluate this.
Hope this helps