Acute Kidney Injury

Acute kidney injury is a different condition from chronic kidney disease, though the two can occur together and the bloodwork can look similar. Understanding the distinction matters because the causes, the treatment approach, and the possible outcomes are quite different. While CKD is a slow, progressive loss of kidney function over months and years, acute kidney injury comes on suddenly, often triggered by a specific event, and in some cases the kidneys can recover substantially or even fully.

If your cat has just been diagnosed with AKI, this page explains what is happening, what treatment looks like, and what to realistically expect. If your cat has CKD and has suddenly become much more unwell, the section on acute on chronic kidney disease may be particularly relevant.

Acute kidney injury, or AKI, occurs when the kidneys suffer a sudden and significant loss of function, usually triggered by a specific damaging event. Unlike CKD, in which damage accumulates gradually over a long period, AKI can develop within hours to days. The kidneys may be structurally normal before the injury and the cat may have appeared completely healthy. After the injury, kidney function can drop dramatically and very quickly.

AKI is serious and requires urgent veterinary care. However, unlike CKD, the damage is not necessarily permanent. In some cases, with aggressive and timely treatment, the kidneys can recover substantially. In others, some permanent damage remains and the cat goes on to develop CKD. In the most severe cases, the outcome is fatal despite treatment.

The key difference from CKD is the possibility of recovery. CKD involves permanent loss of nephrons that cannot regenerate. AKI involves injury to nephrons that may still be capable of healing if the cause is removed and the cat is supported through the recovery period.

Several features help distinguish AKI from CKD, though they can overlap, particularly in cats who have underlying CKD and then experience an acute injury on top of it.

Cats with AKI are usually acutely and severely unwell, often with a sudden onset over hours or days. Cats with CKD typically show a more gradual decline, and owners can often look back and identify earlier subtle signs such as increased drinking, weight loss, or reduced appetite that were present for weeks or months before diagnosis.

In AKI, the kidneys often appear normal or enlarged on ultrasound. In CKD, they are typically small and shrunken, with irregular margins and increased echogenicity. An ultrasound is therefore a useful diagnostic tool when the distinction is unclear.

Anemia is a common feature of CKD, because the kidneys produce the hormone erythropoietin that drives red blood cell production. In AKI, the injury is usually too recent for anemia to develop, so many cats with AKI have normal or even elevated red blood cell counts.

Cats with AKI frequently develop reduced or absent urination, which is less typical of CKD cats who usually produce large volumes of dilute urine. A cat that is not producing urine at all is in a medical emergency regardless of the cause.

AKI can be broadly categorized by where in the kidney system the problem originates.

Before the kidney (pre-renal). When blood flow to the kidneys is disrupted or reduced, the kidneys cannot filter effectively even though the kidney tissue itself is undamaged. Causes include severe dehydration, low blood pressure, heart failure reducing cardiac output, or the use of medications that reduce renal blood flow including NSAIDs and ACE inhibitors. This form is often reversible once the underlying cause is corrected and hydration is restored.

At the kidney (intrinsic). The kidney tissue itself is damaged by a toxin, infection, or other direct insult. This is the hardest form to treat because the nephrons themselves have been injured. Common causes in cats include lily toxicity, antifreeze ingestion, and kidney infections.

After the kidney (post-renal). An obstruction somewhere in the urinary tract, most commonly a stone lodged in the ureter, prevents urine from draining properly. Waste products back up into the kidneys and cause damage. If the obstruction can be relieved quickly, recovery is often possible.

The most common causes in practice:

Lily toxicity is the most critical cause to know about. Most true lilies, including Easter lilies, tiger lilies, Asiatic lilies, and daylilies, are acutely toxic to the kidneys of cats. Even very small exposures can cause severe and potentially fatal AKI. The toxin does not need to be eaten in large quantity: brushing against pollen and then grooming can be enough. Vomiting and lethargy typically appear within hours of exposure, but may appear to resolve temporarily before kidney damage becomes evident. This apparent recovery is misleading. Aggressive treatment must begin as soon as possible; survival rates are close to 100% when decontamination and IV fluids are started within 48 hours of exposure, but drop sharply once significant renal damage has developed. A 2025 study of 112 cats with lily exposure found a 100% survival rate in hospitalized cats versus 86.5% in outpatient cases. If your cat has had any contact with lilies at all, treat it as an emergency and go to the vet immediately without waiting for symptoms.

Antifreeze (ethylene glycol) is highly toxic to cats, who unfortunately find its taste appealing. Unlike lily toxicity, antifreeze poisoning carries a very poor prognosis in cats, with survival rates around 8% after kidney damage has developed. The antidote, fomepizole (4-MP), must be given within a very short window (three to six hours of ingestion) to be effective. This is a true emergency that requires immediate veterinary attention even if the cat seems initially fine.

Kidney infections (pyelonephritis) are bacterial infections of the kidney and the most common cause of AKI in cats who already have CKD. An infected kidney can deteriorate rapidly. Treatment requires a prolonged course of antibiotics, often six to twelve weeks, and sometimes IV fluids during the acute phase. With appropriate treatment, recovery is possible even when bloodwork values look alarming.

Kidney stones (ureterolithiasis) can obstruct the ureter, the tube connecting kidney to bladder, preventing urine drainage and causing the affected kidney to swell and suffer injury. Ultrasound typically shows one enlarged kidney and one small, shrunken one. Treatment depends on the stone location and severity, ranging from IV fluids and diuretics to surgical intervention or placement of a ureteral stent or subcutaneous ureteral bypass (SUB) device.

NSAIDs including meloxicam can cause AKI, particularly in dehydrated cats. Cats metabolize this class of medication poorly, making them more vulnerable than other species. The risk is dose-related and greater when the cat is already dehydrated. If your cat has received an NSAID and develops signs of illness, contact your vet promptly.

Because AKI often comes on quickly, the signs tend to be more dramatic than those seen early in CKD. A cat that was apparently normal a day or two ago but is now clearly unwell warrants same-day veterinary attention.

Signs that may indicate AKI include: sudden onset of lethargy or collapse, loss of appetite that has come on suddenly rather than gradually, vomiting, hunching or obvious discomfort, reduced urination or no urination at all (the latter is an emergency), and in toxic ingestion cases, drooling or neurological signs in the early hours.

The difficulty is that some of these signs are also seen in CKD cats who are having a bad period. The distinction matters most when the cat has no known history of kidney disease and the decline has been sudden and severe.

If there is any possibility that your cat has been exposed to lilies or antifreeze, do not wait for symptoms to appear. Go to the vet immediately.

Your vet will assess several things to diagnose AKI and distinguish it from CKD or other causes of acute illness.

Blood tests. Kidney values including creatinine and BUN will typically be elevated, often dramatically so. AKI cats frequently present with creatinine over 10 mg/dl (880 µmol/L) and sometimes much higher. High phosphorus levels are common. Potassium may be high or low. Calcium tends to be low in AKI, whereas in CKD it is more often normal to high. Metabolic acidosis is typical. A complete blood count checks for infection and assesses whether anemia is present (usually absent in early AKI).

Urinalysis. Assessing the urine helps determine whether the kidneys can still concentrate urine and whether infection, protein, blood, or cellular debris is present. The presence of casts (cylindrical structures shed from damaged kidney tubules) in the urine sediment suggests intrinsic kidney damage. Calcium oxalate crystals in the urine of an acutely ill cat raise suspicion for antifreeze toxicity.

Urine culture. If pyelonephritis is suspected, a urine culture identifies the bacterial organism and guides antibiotic selection.

Cystatin B. A 2024 study found that cystatin B, a urinary biomarker, tends to be elevated in cats with AKI compared to cats with CKD or healthy cats, suggesting it may help distinguish AKI from CKD in some cases. This test is not yet widely available but may become more accessible.

Ultrasound. An abdominal ultrasound is important and should be performed whenever possible. It can identify kidney stones or ureteral obstruction, assess kidney size (AKI kidneys are often normal or enlarged; CKD kidneys are typically small and irregular), and identify other abnormalities that might explain the acute deterioration. In some cases, fluid around the kidneys (perirenal effusion) is visible.

IRIS has developed a separate grading system for AKI, distinct from the four-stage CKD staging system. AKI is graded on a scale of 1 to 5 based on creatinine levels and clinical features.

Each grade is also sub-classified based on whether the cat is producing normal urine, reduced urine, or no urine at all, and whether renal replacement therapy is needed. Cats producing no urine are in a more critical situation than those still producing some.

A critical point that the IRIS guidelines emphasize: the AKI grade is not static. Unlike a CKD stage, which describes a relatively stable situation, an AKI grade represents one moment in a condition that can change rapidly in either direction. A cat in Grade 4 today may move to Grade 2 tomorrow with aggressive treatment, or may deteriorate further without it. Grade at presentation does not define what the outcome will be.

AKI is a hospital emergency. Most cats with AKI Grades 2 through 5 need hospitalization, and the IRIS 2024 guidelines strongly recommend finding a facility with 24-hour monitoring if at all possible, because these cats can change quickly and need frequent reassessment.

Intravenous fluid therapy is the foundation of AKI treatment. IV fluids restore hydration, support blood flow to the kidneys, correct electrolyte imbalances, and promote urine production. Lactated Ringer’s Solution or Plasma-Lyte is typically used. If potassium is dangerously elevated, a potassium-free fluid such as saline may be needed initially, with the fluid type adjusted once electrolytes are more balanced.

The rate and volume of fluids are carefully calculated. More is not better: fluid overload can cause its own serious complications including pulmonary edema and hypertension. Fluid balance should be assessed at least twice daily in hospitalized AKI cats.

One day of IV fluid therapy is not usually sufficient. Most cats need a minimum of several days, and a week or more is not unusual. If a vet suggests euthanasia after only one or two days of treatment because the bloodwork has not improved, it is reasonable to ask whether more time should be given. The IRIS 2024 guidelines explicitly note that lack of improvement in kidney parameters within the first few days does not indicate an inability to recover.

Monitoring urine output is a priority throughout treatment. A cat that is not producing urine is in serious danger. Conversion from oliguric (low urine production) to polyuric (high urine production) AKI is a positive sign. This transition can sometimes be supported with diuretics such as furosemide, though the evidence for diuretics improving outcomes rather than just increasing urine volume is mixed.

Treating the underlying cause is essential alongside fluid support. This means decontamination for toxic ingestions (inducing vomiting or gastric lavage if the exposure was recent), prolonged antibiotic therapy for pyelonephritis, and urological intervention for kidney stone obstruction.

Nutrition must not be overlooked during hospitalization. IV fluids contain almost no calories. Cats who are not eating voluntarily need active nutritional support to avoid the additional complication of hepatic lipidosis. Appetite stimulants and anti-nausea medications should be used if the cat is nauseated.

Blood pressure monitoring is important because hypertension is common in AKI, affecting around 38% of affected cats in one study. Paradoxically, hypertension can also sometimes be caused by excessive fluid delivery, so blood pressure trends alongside other clinical signs help guide fluid management.

Dialysis and renal replacement therapy may be considered for cats who do not respond to conventional treatment and who have creatinine above 5 mg/dl (442 µmol/L) alongside severely elevated BUN. These options are expensive, available at only a limited number of specialist centers, and are typically used as a bridge to allow time for kidney recovery rather than as a long-term solution. They are not within reach for most owners, but are worth knowing about if you are at a facility that offers them.

Prognosis in AKI depends heavily on the cause, the speed of treatment initiation, and the severity of injury.

Lily toxicity carries an excellent prognosis if treated promptly, with survival rates approaching 100% when decontamination and IV fluids are started within 48 hours. Once significant kidney damage has developed and the cat stops producing urine, the prognosis becomes poor without dialysis.

Antifreeze toxicity has a very poor prognosis in cats once kidney damage has developed. The overall survival rate is around 8%. The antidote must be given within a very narrow window.

Pyelonephritis (kidney infection) has reasonable outcomes when treated aggressively. Survival rates of around 57% have been reported in cats with AKI secondary to kidney infection. Cats who recover from AKI caused by pyelonephritis often still have some residual kidney damage.

Ureteral obstruction resolved with appropriate intervention, including SUB device placement, has reported survival rates around 90%.

The IRIS 2024 guidelines reinforce a message that matters: the AKI grade does not define whether recovery is possible. It defines the window of opportunity for recovery. Aggressive, timely treatment gives the kidneys the best chance to heal. High creatinine numbers at presentation, even very high ones, do not automatically indicate a hopeless situation.

Cats with pre-existing CKD can develop AKI on top of their existing disease. This is called acute on chronic kidney disease, or ACKD. It is one of the most common reasons for a sudden, dramatic deterioration in a cat who was previously stable.

The most common cause of ACKD in CKD cats is pyelonephritis. Other causes include kidney stone obstruction, hypertensive crisis, severe dehydration, and in some cases no specific trigger can be identified.

ACKD can look catastrophically bad in terms of bloodwork. The creatinine values are often very high, and owners are sometimes told the situation is hopeless. However, a key consideration is that the acute component of the crisis may be treatable. Resolving the acute injury can return the cat to roughly where they were before the crisis, even if that means returning to the management of established CKD rather than returning to full kidney health.

A 2020 study of 100 cats with ACKD found that 58% survived to discharge from hospital. Pancreatitis was present in over half of the cats. Cats with kidney stones were more likely to survive than those without an identified cause.

When a CKD cat develops what looks like sudden severe deterioration, the question to ask is: what could be causing this acute change? Common culprits to investigate include infection, obstruction, hypertension, and dehydration. Treating the acute trigger often allows a return to stability.

Cats who survive AKI and appear to recover normally are not necessarily in the clear. Even when creatinine and other blood values return to normal, some residual microscopic kidney damage may remain. The IRIS 2024 guidelines recommend that all cats who have experienced AKI be monitored subsequently as if they are IRIS CKD Stage 1 patients, with regular bloodwork and urinalysis even if immediate values look normal.

This monitoring matters because silent, ongoing damage can progress to clinically evident CKD over months to years. Early identification of any developing CKD allows early intervention, which tends to produce better long-term outcomes.

The most preventable causes of AKI are toxic exposures.

Lilies. The simplest and most effective prevention is not to allow lilies into your home. All true lilies including Easter, tiger, Asiatic, and daylilies should be considered dangerous. Peace lilies and calla lilies are a different family and are not nephrotoxic to cats, though they can still cause irritation. If you receive lilies as a gift, dispose of them immediately and wash any pollen from surfaces your cat might groom from. Less than 30% of cat owners are aware of lily toxicity, so sharing this information with other cat owners is genuinely valuable.

Antifreeze. Use pet-safer alternatives where possible, or ensure antifreeze-containing products are stored securely where cats cannot access them. Clean up any spills immediately. Be aware that antifreeze does not only come in obvious containers: some screen wash and coolant products also contain ethylene glycol.

Medications. If your cat is prescribed any NSAID, ensure they are adequately hydrated before and during treatment. Never give human NSAIDs including ibuprofen or naproxen to a cat under any circumstances.

Dehydration. For cats with CKD particularly, maintaining good hydration reduces the risk of dehydration tipping into a renal crisis. Subcutaneous fluid therapy, wet food, and multiple water sources all contribute to this goal.

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