Kidney disease in cats: the slow one you can catch early

Kidney disease does not announce itself. There is no limp, no cough, no obvious bad day. What there is instead is a water bowl that needs filling more often, litter clumps that have quietly doubled in size, and a cat that weighs a little less every time you think to check. Cats compensate so well that outward signs typically appear only once around two thirds of functional kidney tissue has been lost.

That is the frustrating part and also the useful part. The disease is slow, which means there is a long window in which it can be found on a blood and urine test while the cat still looks perfectly well. Cats diagnosed in that window, and managed properly, often live for years with a good quality of life. Cats diagnosed in a crisis usually do not.

What the kidneys actually do

They filter waste products out of the blood, concentrate urine to conserve water, regulate phosphate, potassium and blood pressure, and produce the hormone that tells the bone marrow to make red blood cells. Chronic kidney disease degrades all of that, which is why the illness eventually touches appetite, hydration, blood pressure and even the coat.

The first function to fail is concentration. A kidney that can no longer concentrate urine puts out more water, so the cat loses fluid and drinks to keep up. That single mechanism explains the two earliest signs: more drinking, and larger or more frequent wet patches in the tray.

The early signs, ranked by how useful they are

Notice that everything in the top half is measurable rather than visible. This is why a set of flat kitchen scales, used fortnightly, is one of the better pieces of veterinary equipment a cat owner can buy. Our water intake guide sets out how much a cat of a given weight should be drinking, which makes "drinking more" a number rather than a hunch.

Weight is the earliest honest signal you have. A cat that has lost half a kilogram over six months has told you something, whatever it looks like from the sofa.

Which cats are at higher risk

Age is the dominant factor: prevalence rises sharply past ten, and a large share of cats over fifteen have some degree of disease. Beyond age, the risk factors worth knowing are a family or breed predisposition, notably Persian cats and related flat-faced breeds, where polycystic kidney disease is inherited; Maine Coon and Siamese lines have their own reported predispositions. Previous urinary obstruction, repeated bouts of lower urinary tract disease, chronic dental disease, hyperthyroidism, high blood pressure, and any past episode of acute kidney injury from a toxin such as lily pollen or antifreeze all raise the odds. Our toxic plants guide explains why a single lily can cause irreversible damage in a day.

How it is diagnosed

There is no way to diagnose this from the sofa, and no home test worth relying on. Diagnosis is a blood sample and a urine sample, read together.

• Creatinine rises as filtration falls, but only once substantial function is lost, and it is affected by muscle mass.
• SDMA rises earlier than creatinine and is less affected by muscle mass, which is why it has become the marker of choice for early detection.
• Urea, phosphate and potassium describe how well the rest of the chemistry is holding.
• Urine specific gravity shows whether the kidney can still concentrate. A dilute urine alongside a normal creatinine is often the very first abnormal number.
• Urine protein and blood pressure guide treatment, because protein loss and hypertension both accelerate the disease.

The results are staged using the international IRIS system, which sets the treatment plan and the monitoring interval. Ask which stage your cat is in, and ask for a copy of the numbers so you can compare the next set against them.

What treatment involves

There is no cure and no way to regrow kidney tissue. What there is, and it matters, is a set of measures that slow progression and keep the cat feeling well. Broadly, in order of impact:

1. 1A therapeutic renal diet. Restricted, high-quality protein and controlled phosphate. This is the single intervention with the best evidence for extending survival, and phosphate control is most of the reason. Introduce it slowly, over two to three weeks, because a cat that decides against it will simply stop eating.
2. 2Water, relentlessly. Wet food, several water stations, wide bowls, a fountain if the cat likes moving water. Dehydration makes everything worse and is the commonest reason for a sudden decline.
3. 3Phosphate binders, if diet alone does not bring phosphate into the target range.
4. 4Blood pressure medication, because hypertension is common in these cats and damages the kidney further, along with the eyes.
5. 5Anti-nausea and appetite support as needed. A cat that eats does better than a cat that does not, and appetite loss is treatable. Our guide to a cat that stops eating covers the hour-by-hour side of that.
6. 6Regular rechecks. Every three to six months in stable early disease, more often later, with weight, bloods, urine and blood pressure each time.

Do not put a cat on a renal diet on your own initiative. Restricted protein in a cat that does not need it is not harmless, and the diagnosis has to come first.

Verdict by household

A cat under seven, no signs. Nothing to do beyond a weight record and a routine annual check. Keep lilies out of the house entirely.

A cat aged seven to ten. Ask for a baseline blood and urine screen at the next visit. A normal result now is what makes an abnormal result in three years meaningful.

A cat aged eleven or over. An annual screen including SDMA and urine specific gravity is the standard recommendation, moving to twice yearly past about fourteen. Weigh fortnightly. Our age calculator puts the life stage in context, and the senior signals guide lists what else changes.

A cat already diagnosed. Feed the renal diet consistently, keep water everywhere, keep to the recheck schedule, and weigh at home between visits. Report any drop in appetite early rather than waiting to see whether it settles.

A Persian or a Persian-cross of any age. Ask specifically about polycystic kidney disease, which can be screened for with ultrasound and appears far earlier than age-related disease.

When to call your vet

Book a routine appointment if your cat is drinking noticeably more, urinating more, or has lost weight over recent months, even if it seems well otherwise. Go promptly if appetite has dropped for more than a day, if there is repeated vomiting, if the breath has developed a chemical smell, or if a diagnosed cat becomes lethargic or stops drinking. Treat sudden weakness, collapse, laboured breathing, straining without producing urine, or any suspected lily or antifreeze exposure as an emergency the same hour.

Take numbers with you: current weight against previous weights, how often the bowl is refilled, what is being eaten, and the last set of blood results. Use the portion calculator to check the daily amount is right for the cat's current weight, and the ideal weight tool to judge whether the trend is going the right way.