Hi. What kind of GFR test you used was id eGFR or the Renal GFR Scan? if you eGFR the normal value should be 90 and above. However age and body mass are also factors to consider in analyzing GFR. There was a significant drop in your GFR you really need to be seen by nephrologist for further assess you. Good thing your creatinine is still ok. You should be evaluated to the real cause of kidney disease should be recurrent UTI, post Strep infection, Diabetes or even connective tissue disease like lupus.