Kidney Disease in Cats: The Stages, the Signs, and the Numbers Behind Them

Kidney disease is one of the two or three diagnoses every owner of an aging cat eventually hears about, and for good reason: in a large English mortality study, kidney disorders were the second most common cause of death across cats of all ages, at 12.1%.

This page does something most pages on the subject do not: it shows you where the famous numbers come from, including the one that turns out to be standing on a 1992 magazine review.

How common it actually is, and where the famous figure comes from

You have probably read that 30 to 40% of cats over 10 have kidney disease. Here is what we found when we pulled the thread:

None of this means the big estimates are wrong: a US study that used sensitive early-stage criteria across all ages found CKD in half its randomly selected cats, most of it mild. It means the honest sentence is not one number. Diagnosed kidney disease in the whole cat population runs a few percent; measured moderate disease in screened older cats runs around one in nine; and mild early changes, if you go looking with sensitive tests, are common. Age is the only clearly established risk factor, per the ISFM guidelines and Cornell both.

What CKD is, and why the signs come late

Chronic kidney disease means a sustained loss of kidney function, by convention at least three months. The reason it hides so well is arithmetic: creatinine, the standard blood marker, does not generally rise until about 75% of kidney function is gone. The cat compensates until it cannot.

That is also why the earliest signs are so easy to miss: drinking more and urinating more, because kidneys that are failing lose their ability to concentrate urine first. Dilute urine is one of the earliest lab findings. Later, as waste products build up, come the signs that bring most cats in: weight loss, poor appetite, lethargy, vomiting, a coat gone unkempt, and bad breath, which in kidney disease is described as urine-like or ammonia-like rather than as ordinary bad breath.

When to call the vet: a cat drinking noticeably more, or an older cat losing weight, is an appointment, not an observation to file away. The site rule applies here as everywhere: if something is wrong with your cat, call your vet; out of hours, an emergency clinic.

The IRIS stages, with their actual boundaries

Vets stage CKD on the IRIS system, using fasting creatinine and a newer marker called SDMA, measured at least twice in a stable, hydrated cat. The current boundaries for cats:

Two details from IRIS's own staging document that consumer pages almost never carry. First, SDMA rises earlier: elevations are detectable at roughly 40% function lost, against creatinine's 75%, which is the whole point of the newer test. Second, IRIS has an explicit rule for when the two markers disagree: a cat whose creatinine says Stage 1 but whose SDMA sits persistently above 18 is treated as Stage 2, and the same one-stage bump applies up the scale. If your cat's bloodwork looks contradictory, it may simply be that rule at work.

The staging is completed by two substages: urine protein (a UP/C ratio above 0.4 is proteinuric) and blood pressure, where the current IRIS scheme calls under 140 mmHg normotensive. If you see 150 as the cutoff elsewhere, that page is quoting the older 2016 threshold: IRIS moved it.

The complications worth knowing by name

High blood pressure travels with CKD, and how often depends on who you read: Cornell says about 60% of CKD cats develop it, Merck's owner pages say about 20%. Both are institutional sources, neither cites a study on the page, and we could not resolve the gap, so we give you both. What is not in dispute is that it matters: untreated hypertension damages eyes, brain, heart and the kidneys themselves, which is why blood pressure checks are part of CKD care.

Anemia is the other named companion: the ISFM guidelines put it at 30 to 65% of CKD cats, because failing kidneys make less erythropoietin, the hormone that orders red blood cells.

What management actually changes, with the measured numbers

The cornerstone is unglamorous: food. The ISFM panel strongly recommends a commercial renal diet for every cat with azotemic (Stage 2 to 4) CKD, and this is one of the few places in feline medicine with measured survival numbers attached. In the studies the guideline cites, cats on renal diets versus normal diets survived a median of 633 versus 264 days in one study, and 480 versus 210 in another. Roughly double, in both.

The mechanism is largely phosphate: high blood phosphate is associated with progression, the diets restrict it, and when diet alone is not enough a phosphate binder mixed into food is the next step. Hydration is the other daily lever: wet food, water everywhere, and for advanced cases many owners learn to give fluids under the skin at home every day or three, with their vet's guidance. Blood pressure and urinary protein, when present, are treated with oral medication.

Kidney transplants exist at a few specialist centers in some countries. International Cat Care's honest caveat is worth repeating: the ethics of the donor are real questions, the procedure does not always succeed, and transplanted cats do not necessarily outlive well-managed medical care.

What it costs is the one question we cannot answer with a source we trust: we found no named, dated, verifiable price data for CKD management in any of our four markets, only unsourced content-mill ranges, and we do not publish those.

The prognosis, stage by stage

Cornell's summary is as concrete as this disease gets: cats diagnosed at IRIS Stage 2 average two to three years of survival in many studies; cats diagnosed at Stage 4 average under six months. Which is the argument, made by ISFM, AAFP and AAHA alike, for six-monthly checks in cats over 7 with at least annual bloodwork: the stages you want to find are the quiet ones.

PKD: the inherited kidney disease of the Persian family

Most feline CKD is a disease of age. Polycystic kidney disease is the exception worth its own section: inherited, autosomal dominant, caused by the PKD1 gene, and concentrated in Persians and their close relatives, including the Exotic Shorthair. Fluid-filled cysts slowly replace kidney tissue, silently for years, until the same thirst-weight-appetite signs appear.

The numbers tell a story of testing working: in the UK, screening populations ran 49.2% affected in 2001 and 27.5% of laboratory submissions in 2007, while the 2019 general-practice cohort of 3,235 Persians measured 0.7%. The UK registry, GCCF, has required a negative test for full Persian registration since 2016; the North American registries CFA and TICA carry no such requirement in the documents we read, and UC Davis's own test page still carries the historical ~38% US estimate alongside its note that testing has greatly reduced the frequency.

If you are buying a Persian-family kitten, the practical sentence is one line: ask for both parents' PKD1 DNA test results. Our breed pages carry the full picture.

What to take away

• Chronic kidney disease is staged by IRIS from 1 to 4; survival depends almost entirely on the stage at diagnosis: two to three years on average from Stage 2, under six months from Stage 4 (Cornell).
• A renal diet is the cornerstone with real numbers behind it: the measured diet studies roughly doubled median survival.
• CKD is managed, not cured; acute kidney injury from a toxin such as lily pollen is a different, urgent problem.
• The famous "30 to 40% of old cats" figure traces to a 1992 review; the one modern measured figure is 11.4% of screened cats aged 8 and over (2026 UK study).
• SDMA rises when roughly 40% of kidney function is lost, against creatinine's 75%, which is why it catches the disease earlier.
• The professional bodies recommend six-monthly checks for cats over 7: the stages worth finding are the ones without symptoms.