Chronic kidney disease in cats: diagnosis, IRIS stages and treatment
Chronic kidney disease (CKD) affects a huge number of cats from the age of ten, and it progresses silently for a long time. There is no cure, but early diagnosis and well-managed treatment can give years of good quality of life. This is what you need to know, explained without unnecessary jargon.
What it is and who it affects
The kidney has hundreds of thousands of small filtering units, the nephrons. With age, or after infections, toxins, stones or inherited diseases, some are lost, and those that remain overwork until they cannot cope either. Chronic kidney disease is the result: permanent, progressive damage. It is the most common disease of older cats; it is estimated to affect around one in three cats over ten, and an even higher proportion of those over fifteen. Some breeds are predisposed, such as the Persian and the Exotic because of inherited polycystic kidney disease (PKD), but ordinary moggies also get it very often.
The signs, from early to late
The first, almost always, is that the cat drinks and urinates more, because the kidney no longer concentrates urine properly. Then come loss of weight and muscle mass, especially along the back and hind legs, an erratic appetite, an unkempt coat, vomiting, constipation and dehydration. In advanced stages, bad breath smelling of ammonia, mouth ulcers, weakness, anaemia with pale gums, and lethargy. The problem is that by the time these signs show, the cat has already lost a good part of their kidney function. That is why routine blood tests in older cats matter so much: they pick up the disease before it shows.
How it is diagnosed
With blood and urine tests. In the blood, creatinine, urea and SDMA are measured (SDMA is a marker that rises before creatinine), as well as phosphate, potassium and the haematocrit to check for anaemia. In the urine, specific gravity (a diseased kidney produces dilute urine), protein via the urine protein to creatinine ratio, and sediment and culture to rule out infection. Blood pressure is always measured, because hypertension is very common in cats with CKD and damages the eyes, heart, brain and the kidney itself. Ultrasound shows the size and shape of the kidneys and any cysts, stones or tumours. A single high value is not enough: the diagnosis is confirmed with the cat stable and hydrated, and often by repeating the tests.
The IRIS stages, explained
The International Renal Interest Society (IRIS) classifies the disease into four stages based on creatinine and SDMA in a stable cat. In stage 1, blood values are still normal or nearly so, but there is other evidence of kidney damage, such as poorly concentrated urine or changes on ultrasound. In stage 2, creatinine is slightly raised and the cat usually seems healthy, although they drink more. In stage 3, values are moderately high and symptoms usually appear. In stage 4, the rise is severe and the cat is clearly ill. Each stage is also sub-staged according to proteinuria and blood pressure, which strongly influence prognosis and treatment. Ask your vet which stage and sub-stage your cat is in: it guides everything else.
The renal diet: what extends life the most
This is the measure with the best evidence. Veterinary renal diets have less phosphorus, a moderate amount of high-quality protein, more omega-3, more potassium and B vitamins, and in studies cats eating them live longer and have fewer crises than those staying on their normal food. They are usually recommended from stage 2, or earlier at the vet's discretion. The challenge is getting the cat to accept them: make the switch gradually over two or three weeks, try different brands and textures (there are dry and wet versions), and do not change on a day when they feel nauseous, so they do not develop an aversion. Better a renal diet they eat than the ideal one they refuse, and better their usual food than not eating at all.
Water, the other medicine
A cat with CKD loses a lot of water in urine and tends to become dehydrated, which worsens kidney function. Anything that increases water intake helps: wet renal food instead of dry, water added to food, several water bowls around the home, moving-water fountains if they like them, wide ceramic or glass bowls. In more advanced stages, many vets recommend subcutaneous fluids at home: a small amount of fluid injected under the skin with a fine needle, several times a week or daily. Learning to give them takes a couple of sessions, most cats tolerate it well with a treat, and it is one of the things that most improves how they feel.
What the vet will treat
As well as diet and hydration, treatment is tailored to the stage and to whatever comes up: phosphate binders mixed into food if phosphate stays high despite the diet; blood pressure medication if pressure is raised; medication to reduce proteinuria; treatment for nausea, vomiting and poor appetite, which are the main reason a cat with CKD stops eating; potassium supplements if it is low; treatment for anaemia in advanced stages; and gentle laxatives if there is constipation. It is not a list to do all at once: it is fine-tuning that is reviewed over time. Never give anti-inflammatories or any painkiller without asking, least of all human ones: they damage the kidney, and paracetamol is deadly to cats.
Check-ups and what it costs
A stable cat in stage 2 is usually checked every three to six months; in stages 3 and 4, more often. At each check-up: weight, muscle condition, blood pressure and blood and urine tests. As a guide in Spain in 2026: full renal blood panel with SDMA, 90 to 170 €; urine test with protein to creatinine ratio, 40 to 80 €; blood pressure, 20 to 40 €; abdominal ultrasound, 70 to 150 €. A renal diet costs 30 to 70 € a month depending on how much is wet; supplies for subcutaneous fluids at home, 15 to 40 € a month; and medication, 10 to 60 € a month depending on what is needed. A crisis with hospitalisation and intravenous fluids, 400 to 1,200 €.
How long they can live
It depends a great deal on the stage at diagnosis, on proteinuria, on blood pressure and on how they respond to treatment. A cat diagnosed in stage 2 and well managed can live for years, often many, with good quality of life. In stage 4, the outlook is months or weeks. What makes things worst is dehydration, untreated hypertension, proteinuria and letting the cat go days eating little. What helps most is the boring stuff: diet, water, check-ups and treating nausea before they stop eating.
Prevention and early detection
You cannot stop the kidney from ageing, but you can reduce the factors that speed it up and catch it early. Encourage good hydration throughout life, with wet food and accessible water. Keep away lilies, which cause acute kidney failure even with a little pollen, antifreeze and any anti-inflammatory your vet has not prescribed. Treat urinary infections and dental disease promptly. Do blood and urine tests every year from age seven and every six months from age ten. And if you adopt or buy a Persian, an Exotic or a related breed, ask for the parents' genetic test for polycystic kidney disease.
Chronic kidney disease is the most common disease of older cats and it progresses silently. A yearly blood and urine test from age seven catches it in time, and a renal diet, good hydration and blood pressure control can give them years of good life.
Educational content from Michipedia, translated from the Spanish original. It does not replace the advice of a vet or a feline behaviour specialist. Every cat is an individual.