Urine not concentrated?

I just took my 17 year old cat to the vet, they tested urine and said it was not concentrated. They are sending blood work off for more info - The vet was a little confused b/c he still has a good appetite - but he does only weigh 5 lbs - he is always hungry. He is also having difficulty walking, he sometimes falls, it appears to me that his back legs are giving out on him. Anyone else had this problem?

My kidney girl had that trouble when she had an infection... I talked with the vet and decidied on a potasium supplement for her ...