Your Cat's Kidneys.... and the Metric System.
Two days ago, my last appointment of the day was a new client with her cat, George. The appointment book said that the cat had chronic renal failure (CRF).
The client arrived at 5:30 p.m. She had a copy of the records from her previous veterinary visits. It was a sizable stack, but leafing through it, I finally found some bloodwork results. I recognized the format of the bloodwork as the kind that was performed on an in-house machine. I admit to having some bias when I see these type of reports, because although they’re likely accurate, I always feel that they are inferior to bloodwork performed by a well-known, well-established reference laboratory. Regardless, I looked at the results and noticed that the reference range for some of the parameters were odd. I looked at the top of the page, and quickly discovered why: the bloodwork was performed in the United Kingdom! I then looked at the next line on the report, where they list the cat’s age, weight and color. George was only 1 ½ years old!
I questioned the client. Yes, George was a youngster. I had assumed that he was a senior or geriatric cat, since about 95% of cats with CRF were elderly. CRF in cats less than 10 is uncommon, and less than 2 years old is EXCEPTIONALLY uncommon. George had started drinking a lot of water while the owner was living in England, so she took him in for evaluation. The veterinarian performed some blood and urine tests.
Cats are very good at producing concentrated urine. They’re like desert animals in that regard; they can conserve water if necessary, and produce very strong urine. We describe the urine concentration in medical terms by referring to the urine “specific gravity”. If the kidneys are essentially doing nothing to the urine, the urine concentration will be 1.010 (which we abbreviate “ten-ten”). If the kidneys are working properly, they’ll concentrate the urine to at least 1.040 (“ten-forty”) or higher. A typical young cat that walks through our doors, if we were to test, would have a urine specific gravity of 1.048, or maybe 1.057...something like that.
I looked on George’s report. At one point, it was 1.018. That is indeed dilute, and confirms the owner’s observation that he was drinking a lot. (When you drink a lot, your urine becomes dilute. Beer drinkers know what I’m talking about.) Another urine sample had been reported as being 1.024. A third was 1.032. All of these samples are more dilute than you’d expect in a young cat.
The kidneys’ job is to filter toxins from the bloodsteam and put them into the urine. When the kidneys start to fail and cannot filter properly, the toxin level will start to rise. The cat compensates by urinating more frequently. After all, if the failing kidneys can only put half of the toxins into the bladder, the cat compensates by urinating twice as much. He then drinks twice as much, to avoid getting dehydrated. This is why cats with kidney disease drink a lot and pee a lot. (In other words, cats with kidney disease aren’t peeing a lot because they’re drinking a lot. They’re drinking a lot because they’re peeing a lot. The peeing comes first. The drinking is compensatory.)
George’s urine suggested that he was urinating and drinking a lot. Was the level of his kidney toxins elevated in his bloodstream? I looked at the records. There was some ambiguity.
The two main toxins we evaluate are the blood urea nitrogen (abbreviated BUN) and the creatinine. Both of these toxins elevate, usually in tandem, when the kidneys start to fail. Phosphorus is another toxin we follow, although that usually does not elevate above the high end of the reference range until late in the course of the disease. George’s BUN was normal. His creatinine, however, was above the in-house machine’s reference range. But I admit, I was flustered, because the creatinine was reported in units that were very unfamiliar to me. Those Brits, with their grams and their kilometers and their Celsius degrees… my brain can’t handle it. The units shouldn’t have mattered much. The number was above the reference range. It was elevated. Elevated creatinine and dilute urine usually means CRF. Still, I like my American units. USA! USA!
Still… the BUN was in the normal range, and… the cat is 1 ½ years old! On physical examination, he looked great. Normal eyes, normal ears, clean teeth, normal lymph nodes, heart and lungs sounded fine, abdomen palpated normally. Shiny hair coat. A robust 13 lbs. This cat did NOT look like a cat with CRF.
George’s previous doctor had prescribed Hill’s Prescription Diet K/D, a diet designed for cats with CRF. He has also prescribed Fortekor, the brand name for the drug benazepril. This is a very British thing to do. Benazepril is a drug that we prescribe to cats with CRF if they are urinating out an excessive amount of protein in their urine. One study has suggested that the drug, when given to cats with CRF, will slow the progression of the disease, regardless of whether the cats have excessive protein in their urine or not. The study was not definitive and left more questions than in answered. In America, routine use of benazepril in cats with CRF hasn’t caught on. In the UK, it has.
I told the George’s owner that I didn’t believe that George had CRF. She was shocked, of course. She had already accepted that he did have CRF, and that his lifespan was going to me markedly shortened as a result. I told her I would like to repeat some of the blood tests, using our very-reputable laboratory, so I can evaluate the numbers in units that were familiar to me. She was fine with that. Whatever was necessary, she said.
I obtained blood and urine. To me, the urine looked pretty dark. I usually just wait for the lab to report the results, but curiosity got the best of me, and I put a drop of urine on our refractometer. This is a small instrument that allows you to measure the urine specific gravity pretty accurately in-house. I looked. It was 1.059!!
“Your cat does NOT have kidney disease”, I told her. There is simply no way that a cat has concentrate his urine up to “ten-fiftynine” unless those kidneys are working normally. There’s just no way. I told the owner that I wanted to wait to see what the bloodwork showed, and to see what specific gravity the lab reported, but I felt very confident that George was fine.
The owner was elated. George hated the kidney diet he was on. (The other vet didn’t tell her that there were many other varieties of prescription kidney foods available now; we can usually find at least one or two that the cat enjoys.) George also was difficult to pill, and giving that Fortekor every day wasn’t easy. But most importantly, a normal report gives George a new lease on life. We’d know the results tomorrow.
The next morning the results were in. Urine specific gravity: 1.064. BUN and Creatinine: normal. The verdict: perfectly functioning kidneys.
I don’t fault the vets in the UK for their diagnosis. George DID have one of the notable clinical signs of CRF – the excessive thirst and urination. His urine was dilute, but the bloodwork was iffy, and the age and clinical appearance just didn’t fit. I wouldn’t have been so definitive about the diagnosis, if this was my case initially. I probably would have stressed that we need to recheck the blood and urine in a month or two, and I would have suggested ultrasound to assess the kidneys from a different standpoint. Cats with CRF often have small, dense kidneys on ultrasound, with the normal architecture of the kidneys being altered. This was not performed, or offered. With the bloodwork being so definitively NOT indicative of CRF, I felt that to offer ultrasound now would be a waste of money. It would likely tell me what I already now know: George’s kidneys are fine.