What is Feline CKD?

If your cat has just been diagnosed with chronic kidney disease, you are probably feeling overwhelmed. The word “failure” may be ringing in your ears, and it may feel like you have received a death sentence. Take a breath. For most cats, this diagnosis does not mean the end is imminent. It means you now know what you are dealing with, and knowing gives you the ability to act.

This page explains what CKD actually is, how the kidneys work and what goes wrong in them, why the disease is so common in older cats, what the IRIS staging numbers mean, what the most common causes are, and what the diagnosis realistically means for your cat’s future.

Chronic kidney disease means ongoing, gradual deterioration of kidney function over time. The word “chronic” is the important one: it means this is a long-term condition that develops slowly, not an acute emergency. Many cats live for months to years after diagnosis, and with proper care and some good fortune, the trajectory can often be slowed significantly.

The older term for this condition was chronic renal failure, or CRF, and you may still hear it used. The word “failure” is accurate in a technical sense but is deeply misleading in terms of prognosis. Most people hear “failure” and think their cat is about to die. In reality, most cats diagnosed with CKD are not in immediate danger. The current preferred term, chronic kidney disease, better reflects the actual situation: a disease that can be managed, not necessarily a crisis.

“Renal” simply means relating to the kidneys. If your vet uses the word renal rather than kidney, they mean the same thing.

Understanding what the kidneys do makes it easier to understand what goes wrong in CKD and why the resulting symptoms look the way they do.

The kidneys are paired organs sitting in the abdominal cavity. In cats, as in all mammals, they perform several essential jobs simultaneously.

Filtering waste from the blood. Every minute, a significant portion of the body’s blood supply passes through the kidneys. As it does, waste products produced by normal metabolism, including urea and creatinine, are extracted and excreted in urine. When the kidneys cannot do this effectively, these waste products accumulate in the blood. This buildup is called uremia, and it is responsible for most of the symptoms CKD cats experience.

Regulating fluid and electrolyte balance. The kidneys control how much water the body retains and how much is lost in urine. They also regulate the balance of electrolytes including sodium, potassium, calcium, and phosphorus. When this function is disrupted, cats become prone to dehydration, and electrolyte imbalances develop that affect muscle function, nerve transmission, and overall wellbeing.

Concentrating urine. Healthy cat kidneys are remarkably efficient at conserving water by producing highly concentrated urine. This is an evolutionary adaptation from desert-dwelling ancestors. CKD cats progressively lose this ability, producing large volumes of dilute urine and needing to drink more to compensate.

Producing hormones. The kidneys produce erythropoietin, the hormone that stimulates the bone marrow to produce red blood cells. When this function declines, anemia results. The kidneys also produce renin, which plays a role in regulating blood pressure, and contribute to the activation of vitamin D, which affects calcium and phosphorus balance.

The basic working unit of the kidney is the nephron. Each healthy cat kidney contains around 170,000 to 190,000 nephrons. Each nephron contains a glomerulus, a tiny knot of blood vessels that acts as the primary filter, and a tubule, a fine tube through which the filtered fluid flows and from which water and useful substances are reabsorbed before the remaining waste is passed as urine.

The glomerular filtration rate, or GFR, measures how effectively these nephrons are doing their job. It is the most accurate measure of overall kidney function but is difficult to assess directly in clinical practice, which is why the indirect markers creatinine and SDMA are used instead.

In the vast majority of cats with CKD, the underlying process is a combination of chronic inflammation and fibrosis. As inflammation damages nephrons, the kidney responds by laying down scar tissue (fibrosis) in the affected areas. This is the kidney’s attempt at repair, but the scar tissue does not perform any filtering function. Over time, the proportion of functional nephrons decreases and the proportion of fibrous, non-functional tissue increases. The kidneys gradually shrink and become firm and irregular in texture, which is why a CKD cat’s kidneys often feel small and irregular on examination.

This condition is called chronic interstitial nephritis, or tubulointerstitial fibrosis, and it accounts for around 70% of CKD cases in cats. In most affected cats, the initial trigger for the inflammation is never identified. The process is thought to involve aging-related changes, cumulative low-level insults from various sources, and in some cats, genetic predisposition toward faster renal aging.

The damage is irreversible. Nephrons that are replaced by scar tissue cannot recover. This is why management of CKD focuses on slowing progression and keeping the remaining nephrons functioning as well as possible, rather than on cure.

One of the most common questions from newly diagnosed owners is: how could I not have noticed this sooner? The honest answer is that you almost certainly could not have, and you should not feel guilty about it.

Cats have far more nephrons than they need for normal function. This is called the renal reserve. When nephrons die and function is lost, the remaining nephrons compensate by increasing their individual workload, a process called compensatory hypertrophy. For a remarkably long time, this compensation is sufficient to maintain apparent normality. The cat continues to concentrate urine, filter waste products, and maintain fluid balance without showing any observable signs of illness.

It is not until somewhere between 66% and 75% of nephron function has been lost that the compensatory mechanisms can no longer keep pace, and the first measurable signs begin to appear. This is why CKD is rarely detected until it is already fairly advanced. Even the SDMA test, which is more sensitive than creatinine, cannot detect CKD until approximately 40% of function has been lost. Traditional creatinine testing cannot reliably detect CKD until around 66 to 75% is gone.

By the time your cat’s bloodwork came back abnormal, the disease had very likely been present and progressing slowly for months or possibly years. There was nothing visible to notice, because the kidneys were compensating. The failure to catch it earlier is a biological reality of the disease, not a failure on your part or your vet’s.

CKD is one of the most common diseases of older cats. The prevalence increases steeply with age, estimated at 20 to 50% in cats over 10 years of age and 30 to 80% in cats over 15. Approximately one in three cats will develop CKD at some point in their lives.

The primary reason is aging. The processes of chronic inflammation and fibrosis that underlie most feline CKD are closely linked to the passage of time. Aging kidneys accumulate minor injuries from many sources over a lifetime, and eventually the cumulative burden tips the balance toward disease. This is why CKD is so much more common in cats over 10 than in younger cats, and why prevalence continues to rise with each additional year of age.

Cats appear to be inherently more susceptible to kidney aging than dogs. This is not fully understood, but may relate to their unique metabolism, their obligate carnivore physiology, and the demands placed on their kidneys by their natural diet.

Some breeds appear to have a higher predisposition to CKD, including Maine Coon, Abyssinian, Siamese, Burmese, and Russian Blue. Polycystic kidney disease, a hereditary condition, is most commonly seen in Persian cats and their relatives.

Male and female cats are affected at broadly similar rates, though male cats may be diagnosed at a slightly younger age on average.

Once CKD is diagnosed, the next question is: how advanced is it? The International Renal Interest Society (IRIS) has developed a standardized staging system that most vets now use. It provides a common language for describing severity and guides treatment decisions. The 2023 IRIS guidelines are the current standard.

Staging is based primarily on creatinine and SDMA measured in a stable, well-hydrated cat on at least two occasions. Staging should not be done when a cat is acutely ill, dehydrated, or in crisis, because these states artificially elevate the numbers and give a misleading picture of the underlying situation.

The four IRIS stages

SDMA is assessed alongside creatinine. SDMA can detect CKD earlier than creatinine and is less affected by muscle mass, which matters in older or thin cats where creatinine may be falsely low because of reduced muscle. If SDMA and creatinine suggest different stages, the guidelines generally recommend staging according to the higher of the two values.

Substaging for proteinuria. After establishing the base stage, IRIS recommends sub-classifying by the degree of protein in the urine, measured via the urine protein to creatinine ratio (UPC).

Proteinuria indicates damage to the glomerular filter and is an independent risk factor for faster disease progression. Cats with significant proteinuria generally have a less favorable prognosis than those at the same creatinine stage without proteinuria.

Substaging for blood pressure. The IRIS system also incorporates blood pressure assessment. Systolic pressures consistently above 160 mmHg are considered hypertensive and require treatment. Hypertension damages multiple organs including the kidneys themselves, the eyes, the brain, and the heart.

FGF-23 was added to the 2023 IRIS guidelines as a supplementary marker, particularly useful in cats whose phosphorus levels appear normal but who may already have a phosphorus management problem. FGF-23 is a hormone produced by bone in response to excess phosphorus, and rising FGF-23 levels can indicate a need for phosphorus management even before bloodwork phosphorus becomes elevated.

This is one of the most important things to understand when your cat is diagnosed, and the thing most likely to be misinterpreted or not fully explained at the vet visit.

Staging should only be done on a stable, hydrated cat. If your cat was severely dehydrated when the blood was drawn, or was acutely ill, the creatinine and other values will look worse than the underlying reality. This is why up to 30% of cats presenting with creatinine over 5 mg/dl in a crisis setting turn out to be in a lower stage once they are rehydrated and stable. Do not make treatment decisions or prognosis assessments based on numbers drawn from a cat in crisis.

Creatinine is not a linear measurement. A change in creatinine from 1.6 to 2.8 mg/dl represents a much larger real change in kidney function than a change from 4.0 to 5.0 mg/dl. The relationship between creatinine and actual nephron number is not proportional: as creatinine rises, each additional unit of increase reflects a progressively smaller loss of remaining function.

Numbers do not tell the whole story. Some cats with very high creatinine values feel reasonably well and maintain a good appetite, while others with more modest values feel terrible. The correlation between blood values and how a cat actually presents is surprisingly weak. The IRIS guidelines themselves acknowledge that staging does not define prognosis in any individual cat. On Tanya’s support group, there is a long-standing and accurate mantra: treat the cat, not the numbers.

High numbers at initial diagnosis can improve dramatically. Cats presenting in crisis with creatinine over 5 or even over 7 mg/dl have, in many cases, improved significantly once dehydration was corrected and treatment was begun. What looks like Stage 4 in a dehydrated cat can reveal itself to be Stage 2 or 3 once the cat is stabilized. This is why most specialists recommend giving a cat at least two to four days of intravenous fluids before making any prognosis-based decisions.

In most cats, no specific identifiable cause is ever found. The underlying process is chronic interstitial nephritis related to aging, and searching for a primary cause in an older cat usually does not change the treatment approach.

In a minority of cats, a specific cause can be identified, and in those cases, addressing the underlying cause may change the management. The identifiable causes include:

Polycystic kidney disease (PKD). A hereditary condition, most common in Persians and their relatives, in which fluid-filled cysts develop in the kidneys from birth and gradually expand, eventually destroying functional tissue. A genetic test is available. There is a separate post on PKD on this site.

Kidney infections (pyelonephritis). Bacterial infections of the kidney can cause acute kidney injury and, if recurrent or poorly treated, may lead to permanent scarring and CKD. Pyelonephritis is one of the most common identifiable causes of acute deterioration in cats who already have CKD.

Kidney stones (nephrolithiasis/ureterolithiasis). Stones, particularly calcium oxalate stones, can damage kidney tissue directly or obstruct the ureter, causing acute injury that leaves residual scarring. There is a separate post on kidney stones on this site.

Hypertension. Primary high blood pressure can damage the small blood vessels within the kidneys over time, eventually causing CKD. More commonly, hypertension and CKD worsen each other in a cycle: CKD raises blood pressure, and high blood pressure accelerates CKD.

Glomerulonephritis. Immune-mediated inflammation of the glomeruli, the primary filtering structures. Less common in cats than in humans and dogs, but when present it typically causes significant proteinuria.

Toxins. Lilies and antifreeze (ethylene glycol) are the most common nephrotoxic substances encountered in domestic cats. Both cause acute kidney injury that may resolve or may leave permanent scarring.

Cancer (renal lymphoma). Lymphoma affecting the kidneys is the most common feline renal cancer. The kidneys may feel enlarged on palpation and an ultrasound may show abnormal tissue.

Dental disease. Several studies have found an association between severe periodontal disease and the development of CKD, likely through chronic bacterial seeding of the bloodstream and persistent low-grade inflammation. Whether this is causative or simply associated has not been definitively established.

FIP (feline infectious peritonitis). The dry form of FIP can affect the kidneys and cause CKD, particularly in younger cats.

Even when a cause can be identified, the treatment of CKD itself remains broadly the same. Managing phosphorus, blood pressure, proteinuria, hydration, anemia, and quality of life are the central pillars regardless of what triggered the disease in the first place.

In most cases, yes. The prognosis for a cat with CKD is not a single answer: it depends on the stage at diagnosis, whether proteinuria and hypertension are present, whether concurrent diseases complicate management, how proactive the treatment approach is, and to some extent, individual variation in how well a cat adapts to reduced kidney function.

The Boyd et al. survival study, which remains one of the largest sources of data on feline CKD survival times, found the following median survival times from diagnosis:

These are medians: half of cats at each stage lived longer than the listed time, and half lived shorter. The ranges are striking. Some Stage 2 cats in that study lived over 3,100 days after diagnosis (more than eight years). Some Stage 4 cats, despite an average survival of 103 days, lived over 1,900 days. These are not outliers that should be dismissed; they represent real cats who did well despite discouraging numbers.

Stage 4 numbers at initial presentation in a dehydrated or acutely ill cat should not be used to make prognosis decisions. As noted above, many such cats rehydrate into a lower stage. For Stage 4 cats who are genuinely stable after rehydration, the prognosis is more guarded, but it is not uniformly hopeless, and most specialists recommend a genuine trial of treatment before conclusions are drawn.

Proteinuria and hypertension, where present, worsen the prognosis at any stage. A Stage 2 cat with significant proteinuria has a meaningfully shorter median survival than a Stage 2 cat without it. This is why controlling these factors matters so much.

The most important message, and one that is supported by both the research and the lived experience of thousands of owners who have managed CKD cats: proactive treatment makes a real difference. Cats whose CKD is actively managed with appropriate diet, phosphorus control, blood pressure control where needed, hydration support, and management of symptoms like nausea and anemia consistently do better than those who receive minimal intervention. The diagnosis is not the end of the story. It is the beginning of a management journey that for many cats is measured in years.

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