@cmarouet, I am dealing with a similar situation. Dahalia, a nine MONTH old kitten, has a creatine of 2.4, BUN is normal on Saturday, November 9, 2016. In November 2015, it was 2.0. Historically, her urine has been diluted.
The vet couldn't get a urine sample but an ultrasound and urine analysis are scheduled for this Friday. She also being retested for FIV/FeLK (having previously tested negative). I don't know why but I mention that just in case.