Hello, my GFR went down as yours did. I had formally been drinking a gallon of water a day while I was on immunotherapy. Since I've been off for 4 months I laxed a bit. My egfr went down again so they suggested I see my primary and start drinking a gallon of water a day again. PC did an ultrasound on my kidneys and and I was back up to the -50s and the ultrasound showed no damage to the kidneys, although my egfr was in the '40s off and on for the last year. Hopefully hydration will help you also