Kidney Disease in Cats: What the Numbers Actually Mean

Kidney Disease in Cats: What the Numbers Actually Mean

Kidney disease is the leading medical cause of death in cats. Across the general population it accounts for about 12.1% of deaths, second only to trauma, which mostly kills young cats. In some breeds it is roughly double that.

It is also the disease that hides best, and understanding why is most of what an owner needs.

The Kidney Hides Its Own Damage

A nephron is one microscopic filtering unit. A cat has hundreds of thousands per kidney, and it cannot grow new ones. As nephrons are lost, the survivors compensate by working harder, so function looks normal long after a great deal of it is gone.

The order in which things become detectable matters more than any single number:

That top-to-bottom sequence is the single most useful fact in this article. The urine changes before the blood does. A blood panel that came back normal in a twelve year old cat is reassuring, but it is not the same as a healthy kidney.

Chronic kidney disease (CKD) means kidney damage or reduced function present for months rather than days. It is not reversible. In cats it is classically tubulointerstitial, meaning scarring of the filtering tubes and the tissue between them, rather than damage to the filters themselves.

How Common It Actually Is

The 80% figure is the top of a wide range and should not be read as precise. What it establishes is the shape of the problem: chronic kidney disease is close to an expected part of feline aging in a way it simply is not for dogs.

IRIS Staging, and What the Substages Add

The International Renal Interest Society (IRIS) is an independent non-profit that publishes the staging system almost every vet uses. Two things about it are commonly misunderstood.

First, staging happens after a diagnosis, not instead of one. IRIS says outright that a raised creatinine or SDMA alone is not diagnostic of kidney disease. Second, it requires fasting samples on at least two occasions, in a cat that is hydrated and stable.

Stage 1 means a normal creatinine plus some other renal abnormality: dilute urine with no non-renal explanation, abnormal kidneys on palpation or imaging, protein in the urine, or creatinine climbing across serial samples. That last one is worth pausing on. A rising trend inside the normal range is itself evidence of kidney disease.

Where the two markers disagree, IRIS says stage and treat at the higher level.

The two substages are where the useful detail lives

Proteinuria. The UP/C ratio is urine protein measured against urine creatinine, which corrects for how dilute the sample happens to be. In cats, under 0.2 is non-proteinuric, 0.2 to 0.4 borderline, over 0.4 proteinuric. Dipstick tests are explicitly called out as insensitive.

Blood pressure. Under 140 mmHg systolic is normotensive, 140 to 159 prehypertensive, 160 to 179 hypertensive, 180 and above severely hypertensive. The risk being measured is damage to what IRIS calls target organs: brain, eyes, heart and kidneys. The one owners actually witness is retinal detachment and sudden blindness.

Substaging is the difference between "your cat has stage 2 kidney disease" and "your cat has stage 2 kidney disease that is leaking protein and running a blood pressure of 175." The second is a materially worse situation, and separately treatable.

SDMA, Honestly

SDMA (symmetric dimethylarginine) is a small molecule that every cell with a nucleus produces at a steady rate while recycling proteins, and that the kidneys clear almost entirely. Steady production plus renal clearance means blood levels rise as filtration falls.

The advantages are real. It rises with as little as a 25% drop in filtration, against 75% for creatinine. And unlike creatinine it is not meaningfully affected by muscle mass, which matters enormously in cats: creatinine is generated from muscle, cats with kidney disease lose muscle, so a thin old cat can post a normal creatinine that is really a falsely reassuring one.

The limitations are equally real and get much less airtime:

• It is a proprietary assay. IRIS states plainly that its recommendations rest on published work using one manufacturer's technology, that the methodology has not been standardized, and that it is not known whether other assays give equivalent results.
• Specificity is the weak point. Reported feline sensitivity runs 76% to 94% against creatinine's 71% to 88%, but creatinine's specificity is 94% to 96% against SDMA's 75% to 76%. More cases found, more false alarms raised.
• An independent review is unimpressed. A 2023 appraisal for Veterinary Evidence concluded that most studies analyzed do not demonstrate SDMA is superior to creatinine for assessing filtration in cats with kidney disease, grading the evidence "moderate."
• Non-renal causes exist, including lymphoma, and transient rises in hyperthyroid cats that normalized after treatment.

The fairest summary comes from the ISFM consensus guidelines: SDMA cannot currently be recommended as a single screening test for kidney disease. It is a genuinely useful second data point, and it helps most in exactly the cat where creatinine is least trustworthy.

Why the Urine Test Is Not Optional

Urine specific gravity (USG) measures how concentrated the urine is, which is a direct read on how hard the kidney is working to conserve water.

A dehydrated cat with normal kidneys should produce urine above 1.035. Failure to concentrate in the face of dehydration points at the kidney. Concentrated urine in a cat with normal bloodwork is generally considered incompatible with substantial kidney disease.

The practical version: blood tells you how much function is left, urine tells you how hard the kidney is having to work to deliver it, and the urine changes first. The urinalysis is also the part owners most often decline on cost, which is unfortunate given it is the part that moves earliest.

Two honest cautions. Healthy cats eating mostly dry food and drinking little can produce readings above 1.050, and two to three fold variation has been recorded within two hours. Occasionally a cat with confirmed kidney disease still concentrates above 1.035. It is interpreted alongside hydration status, never alone.

What Owners Actually Notice, and Why It Is Late

Early on, nothing. Then drinking more and producing bigger clumps, which is the two-thirds-lost point arriving. Later: weight loss, sleeping more, and a coat that looks unkempt because the cat has stopped grooming. Later still: poor appetite, vomiting, mouth ulcers, dehydration, bad breath. Sudden blindness or disorientation if blood pressure has gone unchecked, and pale gums if anemia has set in.

Three things compound to make this late:

1. Biology. Surviving nephrons hypertrophy and work harder, so function reads normal until roughly three quarters is gone.
2. Cat behavior. Drinking and urinating more are close to invisible in a cat with outdoor access, a covered tray, or housemates whose clumps look the same.
3. Attribution. Weight loss and sleeping more get filed under old age.

Treatment: What the Evidence Supports

Renal diets, with the fine print

This is the strongest evidence base in feline kidney disease, and it is weaker than the marketing suggests. Three studies do the work:

Read that table in the right direction. The only randomized trial showed fewer uremic crises and fewer kidney-related deaths, and did not show cats living longer overall. The dramatic survival figure everybody quotes comes from the study with the weakest design.

What is doing the work is probably phosphate restriction rather than protein restriction. ISFM describes phosphate restriction as thought to be mainly responsible for improved longevity, while moderate protein restriction is thought to help reduce signs of uremia, with little evidence that it alone affects progression. These diets are also sold by pet food companies, which is worth holding in mind when reading any of it.

Phosphate, blood pressure, and protein

Phosphate. Diet first. If phosphate stays above 1.9 mmol/l, add an enteric binder, a powder mixed into food that traps phosphate in the gut. IRIS targets below 1.5 mmol/l generally, with realistic post-treatment targets of under 1.6 in stage 3 and under 1.9 in stage 4. Monitor every 4 to 6 weeks until stable.

Blood pressure. Treat if systolic persistently exceeds 160, or immediately where there is organ damage. Amlodipine or telmisartan first line, doubled or combined if needed. Do not start either in a dehydrated cat, because filtration can fall sharply. IRIS notes there is no evidence that lowering dietary sodium reduces blood pressure.

Protein leak. Benazepril and telmisartan both reliably reduce proteinuria. Neither has been shown to make cats live longer, which ISFM attributes partly to underpowered trials. IRIS is equally blunt that for borderline-proteinuric cats there is at present no evidence that anti-proteinuric drugs slow progression.

What does not work

The ISFM consensus is unusually direct about this, and it is worth repeating because these things are still sold:

• Anabolic steroids: efficacy information lacking, hepatotoxicity reported, not recommended.
• Stem cell therapy: pilot studies have not demonstrated benefit, some techniques cause adverse effects, not recommended.
• Calcitriol: has not shown the benefits in cats that it shows in dogs.
• Azodyl: a double-blinded controlled trial found that sprinkling the capsule contents into food, which is how owners actually give it, does not reduce azotemia in cats with stable kidney disease.
• Sodium restriction: evidence of benefit generally lacking, and very restricted intake may be harmful.
• Chinese rhubarb, prebiotic and probiotic combinations: no beneficial effects shown.

Polycystic Kidney Disease

PKD, more precisely autosomal dominant polycystic kidney disease (ADPKD), means fluid-filled sacs form in the kidneys from birth and slowly enlarge, crowding out working tissue. Autosomal dominant means one copy of the faulty gene is enough, and an affected parent passes it to half its kittens.

The cause is a single letter change in the PKD1 gene, c.10063C>A, which creates a premature stop signal so the cell builds a truncated version of a protein called polycystin-1. You will see the same variant written several other ways in the literature, because different papers use different reference sequences. They are the same thing.

A terminology point that matters: for a dominant condition there are no carriers. A cat with one copy is an affected cat, not a carrier. Penetrance is reported as 100%.

Two copies are thought to be lethal before birth, which is why almost every affected cat has one. Most of the literature says no homozygotes have ever been found, though a 2009 Italian study reported two alive at three months, so state that as near-absolute rather than absolute.

The success story

Surveys from the 1990s and 2000s put PKD in Persians somewhere around 36% to 50% in most countries. Different reviews give different figures for the same countries, because the studies used different methods, different sample sizes, and in some cases screened only cats already suspected of having the disease.

Then the DNA test arrived. Submissions to the UK laboratory that runs it went from 29% positive in 2005 to 10% in 2009. A 2019 study of 3,235 UK Persians under veterinary care recorded the disease in 0.7% of them. And when researchers genotyped 11,036 cats across 90 breeds, the variant turned up in none of the 118 Persians tested.

That last figure is strong evidence of collapse rather than proof of eradication, since zero out of 118 is still consistent with a true frequency of a few percent. And "eradicated" would be the wrong word anyway. The best current data comes from Japan, where 61,968 cats were genetically tested alongside 110,325 insured cats: carrier frequency across 14 breeds fell 42.6% between 2019, when large-scale kitten testing began, and 2022, with Persians down 38.8% and Scottish Folds down 49.5%. But there was no significant fall in British Shorthairs, Minuets or Munchkins. And a 2022 survey in western Mexico found 25.8% of Persians still carrying the variant.

So the honest version is a genuine success story with an unfinished ending. Where testing is systematic, a dominant variant with a reliable causal test is being driven steadily down. Where it is not, the variant is still propagating in a breed that has had access to the test for twenty years.

Where it stands now

The best current cross-breed data comes from 1,281 cats genotyped at a Tokyo veterinary hospital regardless of history or breed. Overall 1.8% carried the variant.

Treat the Exotic Shorthair row with care: one cat either way moves it by seventeen points. And note the random-bred row, because it makes a point worth stating plainly. PKD is not purely a pedigree problem.

Screening

Cysts are present from birth but are not reliably visible on ultrasound until about 10 months of age, which is why that is the recommended screening age. Sensitivity is 75% at 16 weeks and 91% at 36 weeks. Clinical signs appear at an average of about 7 years, with a reported range of 3 to 10.

DNA and ultrasound answer different questions. DNA tells you whether the gene is there, at any age including newborn. Ultrasound tells you the burden of disease. They disagree more often than you would expect. In the Japanese testing study, only 77.8% of cats diagnosed with cystic kidneys carried the conventional PKD1 variant, meaning roughly a fifth of affected cats would test clear, and exome and whole-genome sequencing turned up eight further potentially damaging variants in PKD-related genes. Cats have also been found genetically positive with no visible cysts. A negative standard test narrows the risk without closing it, which is also why the Maine Coon case is more complicated than it first appears: a Swiss series of six Maine Coons with polycystic kidneys found none of them carrying the conventional variant at all.

The Mortality Picture, Breed by Breed

This is the reason the hub exists.

Both breeds run at roughly double the population rate, and in both it is the single most common cause of death.

One number needs careful reading. Only 1.93% of living Ragdolls had a kidney disorder recorded, against kidney disease causing 21.18% of Ragdoll deaths. That gap is not evidence of underdiagnosis. The first figure is a one-year snapshot of a mostly young population; the second is a share of deaths. Affected Ragdolls had a median age of 14.63 years. A disease that kills old cats will always look small in a cross-section and large in a mortality table. The correct reading is that kidney disease is not common in Ragdolls at any given moment, but it is what most Ragdolls that die of disease die of.

For the other breeds this site covers: Siamese carry a familial amyloidosis that targets liver and thyroid rather than kidney, and kills between one and seven years of age. The Scottish Fold shows a significant PKD1 signal at 6.9%, though its defining welfare problem remains skeletal rather than renal. The American Shorthair sits at about 3%, real but low and not statistically significant. For the Bengal, no breed-specific renal predisposition appears in any risk list I could verify, and saying so plainly is better than inventing one.

The Preventable Causes

Age-related kidney disease is not preventable in any proven way. Acute kidney injury largely is.

Lilies. True lilies and daylilies cause acute kidney failure in cats and not in dogs, from a water-soluble toxin that has never been identified. Every part is toxic including the pollen, and documented poisonings have followed licking pollen off the coat and drinking vase water. Signs start within hours; kidney failure typically follows within 36 to 72 hours untreated. Peace lily, calla lily and Peruvian lily are not true lilies and cause mouth irritation rather than kidney failure, which is a distinction owners routinely miss. Any suspected exposure is a same-day emergency even if the cat looks fine.

Antifreeze. The minimum lethal dose of undiluted ethylene glycol in a cat is 1.4 mL per kilo, so roughly one to two teaspoons. It is metabolized to oxalic acid, which forms crystals that destroy the kidney tubules. Cats deteriorate faster than dogs, with severe failure at 12 to 24 hours rather than 36 to 72. An antidote exists but the window is short, and once signs appear the outcome is usually fatal.

Human painkillers. Never. Ibuprofen and other NSAIDs block the prostaglandins that keep the blood vessels feeding the kidney's filters open, which in a dehydrated cat is the only thing maintaining filtration. Paracetamol is not an NSAID and is separately catastrophic in cats, destroying red blood cells.

Screening That Actually Helps

Senior means over 10, though some cats warrant the label from 8. The guidelines ask for more than one examination a year in senior cats, making the argument by analogy: a cat's lifespan is roughly five times shorter than a human's, so an annual visit is the equivalent of a person being seen every five years.

Baseline diagnostics at least annually from age 7 to 10, increasing with age, and every 3 to 6 months in very elderly cats or those with multiple conditions. A senior panel should include blood count, biochemistry, urinalysis, total T4 and blood pressure.

And get a baseline while the cat is young. Reference intervals are built from populations, but the number that matters is your own cat's. A creatinine of 1.5 mg/dl is normal for the lab; if your cat ran 0.9 at three years old, 1.5 at eleven means a great deal of function has gone while the result still reads normal. IRIS treats exactly that rising trend as diagnostic in its own right. Without the earlier number, nobody can see it.

One Claim Worth Not Repeating

Large studies do find that cats with periodontal disease are more likely to be diagnosed with kidney disease, and the reported effect sizes are enormous, up to a 35-fold increase in one model. Effect sizes that large in observational cohorts usually signal confounding rather than a huge true effect, and the authors say as much themselves.

Two competing explanations sit in the same data. General anesthesia in the preceding year was itself associated with increased risk in the same analysis, and dental treatment in cats requires anesthesia. And a prospective study of cats undergoing dental procedures found their SDMA went up afterwards, with the authors concluding that longer dental procedures in cats may carry inherent risks of kidney injury, which is close to the opposite of the causal story.

Look after your cat's teeth because dental disease is painful and worth treating on its own account, not because it is proven to save the kidneys.

How Dogs Differ

Kidney disease is far less common in dogs, at 0.5% to 1.0% against 1.0% to 3.0% in cats, and 10% of geriatric dogs at referral centers against 60% of geriatric cats. It also attacks a different part of the organ: feline disease is tubulointerstitial, canine disease is much more often glomerular, damaging the filters themselves and presenting as protein loss.

The causes differ too. Dogs get more infectious and immune-mediated kidney disease, and familial kidney disease in dogs is a list of thirty-plus breeds rather than, as in cats, one dominant variant plus two familial amyloidoses. The IRIS staging thresholds are species-specific, so a stage number is not directly comparable between a dog and a cat.

Sources & Further Reading