Kidney disease in cats: the IRIS stages, what to watch for, and how to know when it is time

Chronic kidney disease is the most common serious illness in cats over the age of ten. Roughly one in three cats over fifteen are affected, and almost half of cats over twenty. By the time most cats are diagnosed, the disease has been quietly progressing for months or years.

This guide is for families who have just been told their cat has kidney disease, or who have noticed something at home that has made them wonder. It explains the IRIS staging system that UK vets use day to day, what each stage looks and feels like for the cat, what treatment can and cannot achieve, and how to recognise the point where the conversation begins to change. The article is reviewed by an RCVS-qualified veterinary surgeon.

What "kidney disease" actually means in cats

The kidneys do a quiet, continuous job. They filter waste from the blood, balance fluid and electrolytes, regulate blood pressure, and tell the bone marrow to make red blood cells. When kidney function falls, all of those jobs become harder to do well.

In cats, the picture is almost always the chronic kind. Acute kidney injury does happen, most often after a toxin (lily ingestion is a classic and almost always fatal), severe dehydration, or a urinary blockage in a male cat. Acute injury is an emergency, and prompt hospital care can sometimes restore full function depending on the degree of damage. Chronic kidney disease is different. It develops gradually as nephrons, the millions of tiny filtering units in each kidney, are lost. By the time signs appear at home, around two thirds of working kidney tissue is usually already gone.

The disease cannot be reversed. It can, in many cats, be managed comfortably for a long time. Most of this guide is about that long management, and the decisions that come at the end of it.

The IRIS staging system

When your vet says your cat is "stage 2" or "stage 3", they are talking about the framework set by the International Renal Interest Society. It is the system used across the UK and most of the world to put structure around how advanced the disease is. The full reference is at iris-kidney.com.

Three numbers do most of the work. Creatinine, a waste product the kidneys clear from the blood. SDMA (symmetric dimethylarginine), a newer marker that rises earlier than creatinine. And urine specific gravity, which tells the vet whether the kidneys are still able to concentrate urine properly. Together, with blood pressure and urine protein loss factored in, they place a cat in one of four stages.

Stage 1 cats look well at home. Creatinine sits within the laboratory's normal range. SDMA may be slightly raised. The cat is often picked up at a senior screening visit rather than because anything seemed wrong. There is real value in catching it here: dietary change at stage 1 can slow progression measurably, and we know what to watch for from this point on.

Stage 2 brings creatinine into the mildly raised range, roughly 140 to 250 µmol/L. Many cats remain bright at home, though some families notice slightly increased thirst, a thinner coat, or marginal weight loss. This is the stage where renal-friendly diets, careful hydration, and routine monitoring start to do real work.

Stage 3 is when the disease usually becomes obvious. Creatinine sits at 250 to 440 µmol/L. Thirst is markedly increased, the litter tray fills faster, weight loss is visible, appetite waxes and wanes, and intermittent vomiting often appears. Subcutaneous fluids at home, additional medications, and more frequent check-ins are usual at this stage.

Stage 4 is advanced. Creatinine sits above 440 µmol/L. Cats are often unwell: poor appetite, nausea, mouth ulceration, weight loss, weakness. Treatment shifts towards comfort and dignity rather than slowing progression.

Two things matter about staging. It is a snapshot, not a verdict; cats can hold a stage for many months or years with the right care. And it is one input among several. Two stage 3 cats can have very different paths depending on age, blood pressure, urine protein, concurrent disease, and how they feel in themselves on any given week.

What signs to actually watch for at home

The IRIS stages are useful for vets. For families, the signs at home are more telling.

The earliest change most people notice is drinking more and weeing more. Bowls drain faster than they used to. The litter tray clumps are noticeably bigger. The cat may start drinking from unusual places, the bath, a glass on the bedside table, water left out in the garden. This is the kidneys struggling to concentrate urine, so the body compensates by taking in more water.

Weight loss in an older cat is never to be filed under "they are just slowing down". Cats with chronic kidney disease often lose muscle even when eating reasonably. A cat that feels lighter to lift, or whose spine and hip bones are easier to feel through the coat, has lost weight that matters.

Changes around eating are common and easy to misread. The cat sniffs at food and walks away. They eat in short bursts. They suddenly prefer one food and reject another. Nausea is one of the most consistent features of advancing kidney disease and one of the most under-treated. If your cat has gone from enthusiastic to ambivalent at the food bowl, mention it.

Coat changes show up as the cat grooms less. A coat that was once smooth becomes scurfy along the spine and hips. This overlaps with arthritis pain in older cats and the two often co-exist.

Bad breath with a slightly ammonia-like edge can develop in mid-to-advanced disease. So can mouth ulcers, which make eating actively painful.

Lethargy is the hardest to pin down. Cats sleep more anyway. The question is whether they are also less engaged when they are awake. The family who says "she still has good days, but the bad days are getting more common" is usually describing exactly what is happening.

None of these are specific to kidney disease. Hyperthyroidism, diabetes, advanced dental disease, and several other conditions of older cats overlap. The blood and urine tests sort them out.

What treatment can do

Treatment depends on stage and on what else is going on. The decisions tend to fall into the same handful of buckets.

Diet is the single intervention with the strongest evidence behind it. A veterinary renal diet, reduced in phosphorus and with adjusted protein levels, has been shown in multiple studies to extend life expectancy and slow progression, particularly from stage 2 onwards. The catch is that cats are notoriously fussy about food changes, and a renal diet a cat refuses to eat does nothing. A vet will usually offer several brands and textures, and the right one is the one the cat will eat.

Phosphate binders are added when blood phosphate sits above target despite diet. High phosphate accelerates kidney damage and is one of the more important things to control.

Anti-nausea medication, almost always maropitant (Cerenia) or mirtazapine, makes a measurable difference to appetite and quality of life from mid-stage onwards. Many cats on long-term anti-nausea look like a different animal within a week of starting it.

Blood pressure control matters more than people realise. Hypertension is common in cats with kidney disease and damages the kidneys, heart, and eyes if left untreated. Amlodipine is the standard treatment and is well tolerated. A senior cat being seen for kidney disease should have its blood pressure measured at every visit; many practices now do this routinely.

Subcutaneous fluids at home become an option in stage 3 and stage 4. The vet teaches you to give a measured volume of warmed fluid under the skin every few days. It is unfamiliar at first, almost everyone can do it within two or three sessions, and for many cats it adds real comfort and time.

Erythropoietin-stimulating agents are sometimes used when anaemia becomes significant in late-stage disease. Appetite stimulants, gabapentin for chronic discomfort, and additional supportive medications all have a place at different points.

When the conversation begins to change

For most cats, this is a long, manageable illness. Years of good time at stage 2 are realistic. Months of comfortable time at stage 3 are common. Even stage 4 can mean a peaceful several weeks at home with the right combination of care.

The point at which the conversation begins to change tends not to arrive as a single event. It arrives as a pattern. The bad days outnumber the good ones. The cat is no longer interested in the things that used to define them: a favourite person, a sunny windowsill, the corner of the sofa, food. Medication that worked before has stopped working. New problems, severe mouth ulceration, intractable vomiting, refusal of all food for more than a day or two, start to stack on top of the long-managed ones.

Some specific signs are worth naming because families often look back and wish they had taken them more seriously at the time.

Persistent inappetence in a cat with kidney disease is serious. Cats cannot go without eating for long; even a couple of days of true refusal is a meaningful clinical sign and warrants a call to your vet.

Severe weight loss, to the point where the spine and hips are sharply prominent, often indicates the body has begun to break down its own muscle to compensate. This is rarely reversible.

Hiding, particularly in places the cat would not usually go, is one of the quieter signals that the cat is no longer comfortable. So is sitting in a hunched, tucked posture for long periods.

Frequent vomiting that doesn't respond to anti-nausea treatment, recurring mouth ulcers, and a sustained drop in interaction usually mean the disease has reached a point where comfort is becoming hard to hold.

How to think about the decision

There is no single right moment for euthanasia in a cat with kidney disease. There is a window, usually weeks long, in which the kindest decision moves from "not yet" to "now, before the next bad day". Most families feel that window before they can describe it.

Our Quality of Life Assessment is a short structured tool that can help. It asks seven questions about appetite, hydration, mobility, hygiene, interest, more good days than bad, and pain. None of those questions answer the decision for you. Together, they often give shape to what you have already been feeling.

When the conversation does turn to euthanasia, home is often the kinder setting for a cat with kidney disease. The journey itself is exhausting for an already-fragile cat, and a quiet, familiar room is where most cats are most themselves at the end. Our home euthanasia service is set up for exactly this, and the line is open seven days a week if you want to talk it through before you decide anything.

References worth reading

• IRIS Staging of Chronic Kidney Disease (2023 revision), International Renal Interest Society.
• Sparkes AH et al. ISFM Consensus Guidelines on the Diagnosis and Management of Feline Chronic Kidney Disease, Journal of Feline Medicine and Surgery, 2016.
• Marino CL et al. Prevalence and classification of chronic kidney disease in cats, Journal of Feline Medicine and Surgery, 2014.
• Quimby J. Update on Medical Management of Clinical Manifestations of Chronic Kidney Disease, Veterinary Clinics of North America, 2016.
• Polzin DJ. Chronic Kidney Disease in Small Animals, Veterinary Clinics of North America, 2011.