Understanding CKD Severity

Quick Answer: A chronic kidney disease diagnosis usually means your cat has already lost most of their kidney function, often two thirds or more, simply because CKD produces no obvious symptoms until that much function is gone. This sounds alarming, but it is normal, not a sign of an unusually advanced or unusually bad case. Vets use a staging system from the International Renal Interest Society (IRIS) to classify severity from blood and urine test results, mainly creatinine and SDMA, with proteinuria and blood pressure used to refine the picture. Staging exists to guide treatment decisions, not to predict how long your cat has left. Numbers taken at diagnosis are frequently distorted by dehydration, infection, or stress, and often improve once your cat is stabilized. The single most useful thing you can do after diagnosis is stop focusing on the stage number and start focusing on how your cat is actually eating, moving, and feeling.

Few sentences land harder than a vet saying your cat has lost most of their kidney function. It sounds catastrophic. In reality, this is the expected picture at diagnosis for the great majority of CKD cats, not a marker of an especially severe case.

Cat kidneys have enormous reserve capacity. A cat can lose roughly two thirds of functioning kidney tissue and show no outward signs at all, because the remaining tissue compensates well enough to keep waste products cleared and the cat feeling normal. Symptoms only start to appear once that reserve is used up, which is precisely why routine bloodwork so often reveals CKD only after significant loss has already occurred. A cat who was fine last month and diagnosed with “75 percent function lost” this month has not necessarily crashed overnight. More often, the disease had already progressed quietly for a long time, and something (an infection, dehydration, or simply reaching a threshold) pushed the cat from compensated to symptomatic.

Some cats manage remarkably well even at low levels of remaining function. Veterinary reference sources note that with appropriate supportive care, cats and dogs can survive for extended periods on only a small fraction of functioning kidney tissue, in some cases in the range of five to eight percent. The number on the lab report describes how much tissue has been damaged. It does not by itself describe how your particular cat will do from here.

Standard bloodwork, specifically creatinine, cannot detect CKD until roughly two thirds of kidney function is already gone. SDMA, a newer blood marker introduced in the mid 2010s, can pick up kidney dysfunction somewhat earlier, in some cases around the 40 percent mark, but even that leaves a large window during which the kidneys are already compromised and nothing shows up on a standard panel.

A smaller number of cats are caught earlier, usually because they have annual senior bloodwork done as a matter of routine rather than because they are showing symptoms. This is one of the strongest arguments for yearly (or twice yearly, for cats over ten) wellness bloodwork even in cats who seem perfectly healthy. It will not prevent CKD, but it can catch it before advanced loss has occurred.

A 2024 study published in Scientific Reports looked at the serum metabolome of cats with early versus late stage CKD and found measurable metabolic differences between the two groups, reflecting the ongoing search for biomarkers that could flag kidney disease earlier than creatinine or SDMA currently allow. Earlier detection remains an active area of veterinary research rather than a solved problem, which is part of why so many cats are still diagnosed well after significant function has already been lost.

Older cats, and older literature, often use the term chronic renal failure rather than chronic kidney disease. The word “failure” understandably frightens people, since it suggests the kidneys have stopped working entirely and there is nothing left to be done.

That is not what the term means clinically. CKD is now the preferred term in the veterinary literature because it more accurately reflects what is actually happening, an ongoing, gradual, largely irreversible decline in kidney function that in most cases can be managed for a meaningful length of time rather than an abrupt shutdown. Some vets use “renal insufficiency” for earlier, milder cases specifically to avoid the word “failure” altogether. Whatever term your vet uses, the more useful word to hold onto is chronic. Chronic means ongoing and manageable, not necessarily imminent.

Vets assess kidney function through a combination of blood tests, urine tests, and sometimes imaging. None of these tools tells the whole story on its own, which is part of why staging exists as a structured way of pulling several pieces of information together.

Blood tests measure how well waste products are being filtered out of the bloodstream. The main markers are creatinine and SDMA, both of which rise as filtration capacity drops. These results can be distorted by dehydration, which concentrates the blood and makes kidney values look artificially high, and by concurrent illness such as infection, which can push values up independent of true kidney damage. Correcting the dehydration or treating the infection often brings the numbers down without any change in underlying kidney function.

Urine tests typically look at how concentrated the urine is (urine specific gravity), whether there is excess protein in the urine (proteinuria), and whether there are signs of infection. CKD cats characteristically produce dilute urine, because damaged kidneys lose the ability to concentrate it properly. Infections in CKD cats do not always show up clearly on a standard urinalysis, so a clean urine test does not fully rule one out.

The International Renal Interest Society (IRIS) publishes the staging framework most vets now use to classify CKD severity. It divides CKD into four stages based primarily on creatinine and SDMA, then refines each stage with substages for proteinuria and blood pressure.

Two important ground rules apply before staging is meaningful. First, staging should only happen after CKD has already been diagnosed, using values from a stable, hydrated cat, ideally confirmed across two separate readings rather than a single blood draw. Second, staging cannot be applied reliably to a cat who is currently unstable, severely dehydrated, or in crisis, because the numbers in that state do not reflect the cat’s true baseline kidney function. In practice, many cats are staged off a single visit’s bloodwork, which the IRIS guidelines themselves note is not the ideal approach.

Staging by Creatinine

There is genuine disagreement in the veterinary literature over exactly how much function corresponds to a given creatinine level, and the estimates above should be read as approximate ranges rather than precise measurements. What matters more than the exact percentage is the direction of change over time and how your cat is functioning at that level, not the number in isolation.

Stage 1 deserves a separate note. Creatinine values in this range are within the normal reference range at most laboratories, so a Stage 1 classification only applies to a cat who already has other evidence of kidney disease, such as SDMA elevation or a structural abnormality seen on imaging, not simply to any cat with a normal creatinine reading.

SDMA is considered more sensitive to early kidney dysfunction than creatinine, partly because it is less affected by muscle mass. A cat with reduced muscle, common in older cats, can have a falsely low creatinine reading that masks how much kidney function has actually been lost, whereas SDMA is not distorted in the same way. Current IRIS guidelines recommend staging a cat according to whichever marker, creatinine or SDMA, indicates the more advanced stage, since either one on its own can understate the picture.

It is worth noting that SDMA’s sensitivity comes with a tradeoff. IRIS itself has acknowledged that the specificity of SDMA, meaning how reliably an elevated reading actually indicates CKD rather than something else, is not yet fully defined at a population level. A single elevated SDMA reading is a reason to monitor and retest, not necessarily an immediate diagnosis on its own.

Once a stage is established, IRIS further substages based on the urine protein to creatinine ratio (UPC), which measures how much protein is leaking into the urine. Higher proteinuria is associated with faster disease progression and shorter survival times in published studies, which is why controlling it, often by managing blood pressure, is a meaningful part of treatment even in early stage cats.

A single elevated UPC reading is not conclusive. Blood in the urine, infection, and inflammation can all produce a falsely elevated result, which is why the standard approach calls for confirming proteinuria across multiple samples collected over at least two weeks before drawing conclusions.

High blood pressure is common in CKD cats and is assessed separately, since it carries its own risks, particularly to the eyes and nervous system, independent of kidney damage itself. Sustained high readings over one to two weeks, rather than a single stressed in-clinic reading, are what actually inform this substage, since blood pressure measured at the vet can be temporarily elevated simply from the stress of the visit.

If your cat was diagnosed while sick, dehydrated, or in crisis, the numbers from that visit are very likely worse than your cat’s true baseline. Several things commonly inflate kidney values at the point of diagnosis, including dehydration, an untreated urinary or kidney infection, uncontrolled high blood pressure, and kidney stones. Once these are addressed and your cat is stable, a repeat set of bloodwork often looks meaningfully better, not because the underlying kidney disease has been cured, but because the numbers finally reflect reality rather than a temporary crisis on top of it.

This is one of the most important things to understand in the days immediately after diagnosis. A frightening creatinine value at an emergency visit is not a reliable long-term prognosis. It is a snapshot taken under the worst possible conditions.

Research looking back at cats with stable CKD has found that a single creatinine reading at presentation is a surprisingly poor predictor of how long a cat will actually go on to live, explaining only a small fraction of the variation in survival time. Clinical condition, in other words how the cat is actually eating, moving, and behaving, tends to matter more for short-term prognosis than a single lab value.

This is the reasoning behind a phrase used often among longtime CKD caregivers, treat the cat, not the numbers. The stage on paper is a starting point for a treatment plan, not a verdict. Some cats with high numbers do far better than expected. Some cats with comparatively mild numbers struggle more. Watching how your cat is actually doing, and adjusting treatment in response, generally tells you more than watching the stage label alone.

Cats in Stage 1, Stage 2, or the lower end of Stage 3 typically do not need daily supportive fluids and can often be managed well with a handful of targeted treatments, addressing things like nausea, constipation, or elevated phosphorus as they come up, plus management of proteinuria or high blood pressure if present. Getting your cat eating well is the priority at every stage, regardless of number.

Cats at the upper end of Stage 3 or in Stage 4 usually need more active management, often including regular subcutaneous fluids to prevent dehydration, and are more likely to develop complications such as anemia or metabolic acidosis that need their own treatment. A high stage at diagnosis is not a reason to give up. Before accepting those numbers as your cat’s true baseline, it is worth confirming that dehydration, infection, hypertension, or kidney stones have been ruled out or treated, since any of these can make a cat appear far more advanced than they actually are.

Nobody can reliably predict how quickly an individual cat will move through the stages. Some cats’ bloodwork stays remarkably stable for months or years at a time. Others progress more quickly. This uncertainty cuts both ways, and it is generally worth giving appropriate treatment a real trial, typically at least a couple of weeks, before concluding it is not helping.

The term end stage renal disease is used inconsistently, which causes a lot of unnecessary fear. Some vets use it specifically to describe a cat who is actively declining, no longer responding to treatment, and likely close to the end. Others use it loosely to describe any cat whose bloodwork sits at Stage 2 or above, even though many cats at those stages go on to live for a long time with appropriate management.

If your vet uses this phrase, it is worth asking directly what they mean by it in your cat’s specific case. A vague reference to “end stage” based purely on a lab value, without confirming the cat is actually declining clinically, is not the same thing as a cat who has genuinely run out of options.

My vet said my cat has lost most of their kidney function. Does that mean it’s hopeless?

No. Most cats are not diagnosed until a large majority of kidney function is already gone, simply because CKD produces no visible symptoms before that point. This is the normal pattern for the disease, not a sign that your cat’s case is unusually severe.

Why did my cat’s numbers look terrible at the emergency vet but improve afterward?

Dehydration, infection, uncontrolled high blood pressure, and kidney stones can all inflate kidney values temporarily. Once these are treated and your cat is stabilized, bloodwork often looks meaningfully better, reflecting your cat’s true baseline rather than a crisis on top of it.

Is IRIS staging the same as a prognosis?

Not directly. Staging is designed to guide treatment decisions, not to predict survival time. Clinical condition, meaning how your cat is actually eating and feeling, has been shown to be a better short-term predictor than a single lab value.

Can a cat’s stage improve over time?

Yes, particularly if the diagnosis numbers were inflated by a treatable factor like dehydration or infection. True kidney function does not regenerate, but the numbers reflecting it can improve once complicating factors are addressed, and can also remain stable for extended periods with appropriate management.

Should I trust a single blood test result?

Be cautious about drawing firm conclusions from one reading, especially if your cat was unwell or dehydrated at the time. Guidelines call for confirming values across at least two readings in a stable, hydrated cat before treating a stage as reliable.

International Renal Interest Society (2023). IRIS Staging of CKD.

International Renal Interest Society (2023). IRIS Treatment Recommendations for CKD in Cats.

Van Vertloo, L. (2026). Renal Dysfunction in Small Animals. MSD Veterinary Manual.

Grauer, G.F. (2015). Feline Chronic Kidney Disease. Today’s Veterinary Practice, 5(2), 36-41.

Mortier, F., van Leeuwenberg, R., Daminet, S., & Paepe, D. (2023). Determination of Age-Specific Reference Intervals for Selected Serum and Urinary Biomarkers in Elderly Cats. Journal of Feline Medicine and Surgery, 25(11).

Syme, H.M., Markwell, P.J., Pfeiffer, D., & Elliott, J. (2006). Survival of Cats with Naturally Occurring Chronic Renal Failure Is Related to Severity of Proteinuria. Journal of Veterinary Internal Medicine, 20, 528-535.

Elliott, J. (2000). Prolonging the Life of the Renal Failure Patient. Waltham Focus, 10(3), 10-14.

Polzin, D.J. (2006). Renal Disease. Delaware Valley Academy of Veterinary Medicine.

Sanderson, S.L. (2025). Overview of the Urinary System. MSD Veterinary Manual.

Scientific Reports (2024). Untargeted Metabolomic Profiling of Serum from Client-Owned Cats with Early and Late-Stage Chronic Kidney Disease. Scientific Reports, 14, 4755.