I'm so very sorry about your pet.
My Maggs came to us drinking lots of water, blood tests we had taken diagnosed kidney problems caused by high blood pressure.
She has six monthly blood pressure tests, yearly blood tests and daily blood pressure meds. When they did a blood test @ six months ago it said that her kidneys were functioning reasonably normal, but she still drinks a lot of water.
I agree with you blood tests can illuminate many problems with our pets.
Once again, so very sorry for you