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I’m a big fan of urinalysis (U/A) because it’s non-invasive (no need for cystocentesis if one is not testing for bacterial infection), and can be a useful tool for not just early diagnosis of kidney insufficiency but also for monitoring progression of kidney failure to some extent.
A U/A will show among other things, whether there is excess protein being spilled in the urine, as well as specific gravity (USG) of the urine. In normal cats, there should never be protein in the urine. For raw-fed cats (who typically have more dilute urine anyway due to the high moisture content of meat), a USG of less than 1.03 or so (combined with clinical symptoms and/or blood values such as elevated BUN and Creatinine) becomes a warning sign because it signals decreased filtering ability of the kidneys.
I get U/As for my cats at least every six months because it’s so easy to just drop off a sample at my vet’s. For $20, I get a sense of my cats’ liver, kidneys, and pancreas health, so it’s well worth it, IMO. I’m even crazy enough to have purchased a hand-held refractometer for around $35 so in between full U/A from my cats’ holistic vet, I can quickly and easily check USG at home for each of my cats on a monthly basis.
You might not be as big a nutter as yours truly, but if you can swing even once a year testing, please consider it. This is even more important after age 7 or 8, although it is not uncommon for cats as young as 2 years of age eating dry food to have failing kidneys. While we’re on the subject, please pretty please do not feed your cat that horrible grain-based dehydrated junk, which is about as far removed from cat prey as can be. For a quick read (I promise!) on why, click here.
When it comes to blood work, it only shows raised levels of Blood Urea Nitrogen (BUN) and Creatinine -- both of which are waste products excreted in the urine -- after at least 70-75% of the kidney function is lost. Rule of thumb - if your cat's BUN is higher than 33 or so, and Creatinine is higher than 1.8 - 2.0 Mg/Dl (varies, so please consult your lab's range), less than 25% of kidney function is left. Note: European and Commonwealth e.g. British and Canadian measurements are different so click here or if previous link is not working, then check Merck Vet Manual link here for conversions to US units.
It’s best to do a SuperChem profile, which gives not only BUN and Creatinine, but also Phosphorus and Potassium which are important to monitor as well.
In addition, since cats with CRF can be prone to anemia, it’s good to get a complete blood count (CBC) which looks at the numbers of white and red blood cells (RBCs). A lower than normal number of RBCs is significant because it often indicates a drop in erythropoiten, a hormone secreted by the kidneys that stimulates RBC production.