A couple of clients recently requested that I post about kidney disease in cats. It's been a while since I have done so.
I previously wrote THIS about renal disease management but it was really just a few words about somebody else's handout that I linked to.
There were two specific questions I wanted to answer for starters.
#1 - Why are the kidneys so trouble-prone in cats? We don't know. Cats, being desert animals, have a huge reserve of kidney function built in, which is important because, unlike the liver which can regrow functional tissue, when kidney tissue dies it is not replaced. In nature, kidneys only need to last long enough to ensure propagation of the species, which as you know can be accomplished several times by cats before they are two years old. So when we see kidney disease in older cats it may just be that it's a part wearing out, like tires on a car.
But we also know certain things can contribute to premature loss of kidney tissue and a decline in function. Untreated bacterial infections (most commonly dental disease) can cause bacteria to enter the bloodstream where they are filtered out by the kidneys and set up housekeeping. Some cats have a genetic kidney defect known as polycystic kidney disease (PKD) that leads to renal failure in middle age - Persians and Himalayans are especially prone to this. Some cats develop renal lymphoma, a type of cancer formerly associated with feline leukemia virus infection but now most commonly seen in cats whose owners smoke. There is also a theory that certain vaccines which use a cell line derived from feline kidneys might be leading to an immune system attack on kidney tissue (we do not use those particular brands of vaccine).
#2 - How much urine do healthy cats normally produce and how can an owner quantify their own cat's urine output in the real world? Normal urine output in the cat is under 50 ml/kg/day. That's about (ok doing math in my head now) 8 oz for an 11 lb cat, more-or-less. Of course with cat's using a litter box it can be hard to tell how much a cat is urinating (volume) and how often, which is one major reason I strongly suggest using scoopable litter.
Urine balls can be easily quantified, and over time you can get a sense of how many times a day your cat urinates and how big the balls tend to be. What's important is that you make not of changes and bring them to your veterinarian's attention. My own cats normally urinate 2-3 times a day, and the bigger cat has bigger urine balls than the smaller one so I can often tell whose is whose.
If you notice that your cat is producing larger urine balls, and more of them, the two things that come to mind that we have to rule out are kidney disease and diabetes. If the urine balls are smaller and more numerous we need to rule out Feline Lower Urinary Tract Disease, which sometimes means an infection is present (but not always).
Here at Cat's Meow Veterinary Clinic we do not measure urine output directly - cats don't tend to go along with such plans. We measure urine specific gravity (concentration) - cats that produce dilute urine are automatically going to produce a higher volume of urine, but the number we track is USG. The exception is in terminal end-stage renal failure where only scant amounts of very dilute urine are produced, right before none at all is produced - cats are typically euthanized before things reach this point.
I hope this sheds some light on the subject. Kidney disease is one of the most common things I manage in my older patients.