Kidney Stones

Quick Answer: Kidney stones in cats are mineral deposits that form inside the kidneys. In cats, the vast majority are calcium oxalate stones, which form in acidic urine and cannot be dissolved through diet alone. They are common in cats with chronic kidney disease, affecting up to 73% of CKD cats in some studies. Inactive stones that are not moving are usually monitored rather than treated. When a stone migrates into the ureter, it can cause a life-threatening blockage. The most effective modern treatments are ureteral stents and subcutaneous ureteral bypass devices, though both are expensive and available only at specialist centres.

Kidney stones, known medically as nephroliths or renal calculi, are mineral deposits that form inside one or both kidneys. They are a recognised complication of feline chronic kidney disease and can cause or worsen kidney damage depending on whether they remain static or begin to move.

The majority of feline kidney stones are calcium oxalate stones. Unlike struvite crystals, which form in alkaline urine and can sometimes be dissolved with dietary changes, calcium oxalate stones form in acidic urine and cannot be dissolved. This limits treatment options considerably.

Many cats with kidney stones have no symptoms at all, and static stones that show no sign of moving are often monitored rather than actively treated. The picture changes when a stone migrates into the ureter, the narrow tube connecting a kidney to the bladder. A lodged stone can block the flow of urine entirely, causing acute kidney injury and a medical emergency.

Newer interventional treatments, particularly the subcutaneous ureteral bypass device, have significantly improved outcomes for cats with ureteral obstructions. These treatments require specialist expertise and carry substantial cost, but success rates are good.

The feline urinary tract consists of two kidneys, two ureters, the bladder, and the urethra. The kidneys and ureters form the upper urinary tract. The bladder and urethra form the lower urinary tract. Understanding this distinction matters because the type of stone and its location determine what treatment is possible.

The medical term for stones in the urinary tract is uroliths or calculi. Urolithiasis refers to the condition of having them. There are two main types seen in cats.

Struvite

Struvite crystals form in alkaline urine. They are typically found in the lower urinary tract (bladder or urethra) and fall under the category of feline lower urinary tract disease. In some cases they can be dissolved with an acidifying diet. They are rarely seen in cats with CKD, who tend toward acidic urine. This page does not address struvite further.

Calcium Oxalate

Calcium oxalate stones form in acidic urine. They are the dominant type found in the kidneys and ureters, accounting for over 90% of feline kidney stones. They cannot be dissolved through dietary changes. The remainder of this page focuses on calcium oxalate stones in the upper urinary tract.

Occasionally, calcified blood clots rather than true stones form in the kidneys. These behave similarly to stones but are sometimes more amenable to medical management.

The presence of kidney stones increases the risk of kidney infections (pyelonephritis) and can contribute to the development or progression of chronic kidney disease.

Many kidney stones are static, meaning they remain in place without moving. These may cause no symptoms and in many cases the owner does not know the stones are there. Static stones are generally monitored rather than treated, as the risks of intervention often outweigh the risks of leaving them in place. Research has confirmed that the presence of static nephroliths in CKD cats does not significantly accelerate kidney disease progression.

Problems arise when a stone becomes active and begins to move. Smaller stones may pass through the ureter on their own, though this is painful and can take weeks or months. Larger stones can become lodged in the ureter, causing what is called a ureteral obstruction or obstructive nephropathy.

The feline ureter is extremely narrow, roughly 0.3mm in diameter. For comparison, the wire used in a standard paper clip is approximately 1mm thick. Any stone large enough to block this passage prevents urine from draining from the affected kidney. Waste products that would normally be excreted build up in the kidney instead, causing acute kidney injury.

Stones in the kidneys are referred to as nephrolithiasis. Stones in the ureter are referred to as ureterolithiasis. If one ureter has previously been obstructed and that kidney has silently ceased to function, the cat may be relying on a single working kidney. A subsequent obstruction in that kidney places the cat in immediate crisis.

This pattern, where one kidney is small and non-functional and the other is enlarged and compensating, is known as big kidney little kidney syndrome. When the enlarged kidney also obstructs, the cat cannot urinate and requires emergency treatment.

Kidney stones tend to form when the urine contains higher than normal concentrations of the minerals that make up the stone. In the case of calcium oxalate stones, those minerals are calcium and oxalate. Several factors can tip the balance toward stone formation.

Urine acidity: Urine with a pH below 6.0 promotes calcium oxalate crystal formation. The trend toward acidifying commercial cat foods, introduced to prevent struvite crystals, is thought to have contributed to the rise in calcium oxalate stones seen in cats over recent decades. Reduced magnesium content in modern cat foods may also play a role, as magnesium inhibits calcium oxalate formation.

Hypercalcaemia: Elevated calcium levels in the blood, particularly ionised hypercalcaemia, are strongly associated with calcium oxalate stones. One study found that cats with ionised hypercalcaemia were over 83 times more likely to have a ureteral obstruction than cats without it. Whether the hypercalcaemia causes the stones or the stones drive the hypercalcaemia is not yet established.

Elevated FGF-23: Increased levels of fibroblast growth factor 23, already associated with CKD progression, have been found to correlate with hypercalcaemia in cats with both CKD and kidney stones.

Reduced water intake: Concentrated urine increases the likelihood that minerals will precipitate into crystals. Cats eating dry food exclusively appear to be at higher risk.

Vitamin B6 deficiency: In some cats, a deficiency of vitamin B6 can contribute to oxalate accumulation and stone formation.

Breed predisposition: Longhaired breeds and certain pedigree cats, including Persian, British Shorthair, Ragdoll, and Scottish Fold, appear to be at increased risk. One study found that being purebred was significantly associated with the presence of kidney stones in CKD cats.

Medication: GS-441524, the active metabolite of remdesivir used in cats with feline infectious peritonitis, has been linked to crystal nephropathy in a small number of cases. The recommendation is to use this medication for the shortest period necessary.

Dietary calcium does not contribute to stone formation. In fact, calcium consumed with food can bind to oxalate in the gut and prevent it from being absorbed, reducing the amount available to form stones. Calcium supplements, on the other hand, may carry some risk and should not be given without veterinary guidance.

Cats are skilled at concealing pain. Many cats with kidney stones show no outward signs, and the stones are found incidentally during imaging for another reason. When symptoms do appear, they may include the following.

• Subdued behaviour, lethargy, or reduced appetite
• Blood in the urine (haematuria)
• Frequent kidney infections
• Changes in urination, either producing more urine than usual or straining to urinate
• Waxing and waning kidney values on blood tests, which may suggest big kidney little kidney syndrome

If your cat is unable to urinate at all, this is a medical emergency. Get to a vet as quickly as possible.

Blood Tests

A sudden and significant rise in kidney values (BUN and creatinine) can indicate that a stone has blocked a ureter. Cats with active kidney stones may also show elevated phosphorus and potassium levels. Around 48% of cats with kidney stones are anaemic, often because of concurrent CKD, infection, or inflammation.

Palpation and X-Rays

Your vet will palpate the kidneys, which may feel tender or asymmetrical. X-rays are a useful first step and can help the vet assess the number, size, and rough location of stones. The difference in kidney size that characterises big kidney little kidney syndrome may be detectable on palpation alone, which will prompt further imaging.

Ultrasound

Ultrasound can identify stones in many cases, though it is not infallible. Calcified blood clots are often not visible on ultrasound. What ultrasound is particularly useful for is assessing the degree of pelvic and ureteral dilation.

Dilation of the renal pelvis is called pyelectasia when mild to moderate and non-obstructive. When dilation is caused by an obstruction, the term hydronephrosis is typically used. Research has found that renal abnormalities on ultrasound, including asymmetry and pelvic or ureteral dilation, should raise suspicion of a ureteral obstruction and prompt consideration of referral to a specialist centre.

Ultrasound findings alone should not determine whether decompression is pursued. Research has found that no preoperative imaging characteristics reliably predict whether kidney function will recover after a ureteral obstruction is relieved.

The appropriate treatment depends on whether the stones are active or static, whether an obstruction is present, and which kidneys are affected. Inactive stones are not normally treated in cats, even those with CKD. Active monitoring via urinalysis, blood tests, and imaging every few months is the standard approach for static stones.

When treatment is required, the options fall into three broad categories: medical management, surgical management, and interventional management.

Medical Management

Medical management is recommended as a first step for 24 to 48 hours in acute cases. During this period, spontaneous passage of the stone is possible. Research indicates it is effective in roughly 8 to 17% of cases of ureteral obstruction.

Intravenous fluids: The cat is hospitalised and given IV fluids to flush the urinary tract. Lactated Ringer’s solution is not used because it contains calcium. Normosol-R is a suitable alternative. Fewer than 10% of true kidney stones pass with diuresis alone, though calcified blood clots respond better.

Diuretics: Furosemide or mannitol may be added to increase the flushing effect. Guidelines recommend that mannitol be used in conjunction with IV fluids.

Ureteral relaxants: Medications including amitriptyline and prazosin are sometimes given to relax the smooth muscle of the ureter, potentially allowing a stone to pass. Prazosin use has been associated with a higher rate of reblocking in some studies, so its use should be guided by the vet based on the individual case.

Antibiotics: A concurrent urinary tract infection is common in cats with ureteral obstructions, so broad-spectrum antibiotics and a urine culture are recommended for all cats in an acute situation.

Dietary changes: Calcium oxalate stones cannot be dissolved with diet, but dietary adjustments can reduce the risk of new stones forming. Most therapeutic kidney diets are appropriate for cats with calcium oxalate stones. Acidifying foods labelled for urinary tract health should be stopped immediately, as these worsen the conditions that lead to calcium oxalate formation.

Pain management: Active kidney stones can be extremely painful. Pain relief should be discussed with your vet as part of any management plan.

Surgical Management

Traditional surgery, including cutting into the ureter (ureterotomy) or opening the kidney to remove stones, carries a complication rate above 30% and a mortality rate in cats of around 18%. Stone recurrence after surgery is common. In almost all cases, an interventional approach is now preferred over traditional surgery.

Extracorporeal shock wave lithotripsy (ESWL), which uses sound waves to break stones into passable fragments, is used in humans and some dogs but is rarely suitable for cats. Because the feline ureter is only 0.3mm in diameter, even fragments smaller than 1mm can cause an obstruction. One cat is reported to have had a successful ESWL procedure at a US veterinary school in 2025, which may indicate the technique is beginning to be used more in cats.

Percutaneous nephrolithotomy (PNLE) is an endoscopic keyhole technique for large stone burdens and has been used successfully in small dogs and cats, though it remains uncommon in feline practice.

Interventional Management: Ureteral Stent

A ureteral stent is a small mesh tube placed inside the ureter. The stent causes the ureter to dilate passively over time, increasing its diameter from approximately 0.3mm to as much as 1.5mm within two weeks. This allows urine to flow around or past stones and, in some cases, permits smaller stones to pass.

Success rates are high, around 96% for stent placement. However, around 22% of cats experience a re-obstruction and may need a replacement stent. Complications include urinary tract infections, biofilm formation on the stent, blood in the urine, and chronic lower urinary tract signs. Some cats with stents appear to be more comfortable when given ongoing pain management, suggesting the stent itself causes low-grade irritation in some cases.

Stents are expensive. Costs in the USA have ranged from around US$8,000 for two stents to US$15,000 for a combined stent and bypass procedure. Replacement stents add further cost. Estimates at the time of initial consultation may understate the eventual total.

Interventional Management: Subcutaneous Ureteral Bypass (SUB)

The subcutaneous ureteral bypass, developed at the Animal Medical Center in New York City, is now generally preferred over stents where the cat is a suitable candidate and an experienced surgeon is available. It does not aim to remove the stones but provides an artificial route for urine to bypass them entirely.

A SUB consists of tubing that runs outside the ureter, connecting the renal pelvis to the bladder. Because it sits outside the ureter rather than inside it, the device can be substantially wider than the ureter itself, around 5 to 6mm in diameter. This means urine flows freely regardless of stone activity in most cases.

To be eligible for a SUB, the renal pelvis must be dilated, which typically means an active blockage is present at the time of surgery. Cats without dilation may be offered a stent instead.

An important advantage of the SUB over a stent is the access port. A small port is implanted under the skin of the abdomen, which can be used to flush the device periodically. Flushing with a solution called tEDTA (tetrasodium ethylenediaminetetraacetic acid) significantly reduces the risk of biofilm formation and infection. One study comparing tEDTA flushing to saline flushing found an infection rate of just 3% with tEDTA, compared to 33% with saline alone.

Current flushing recommendations are at discharge, one week post-operatively, one month post-operatively, and then every three months. Flushing appointments are brief and do not normally require hospitalisation.

The latest generation device, the SUB 3.0, has reduced the kinking seen in earlier versions and requires shorter anaesthetic time. Long-term complication data for the 3.0 device are still being gathered.

Success rates are good. Across studies, the median survival time after SUB placement has been reported at over 800 days. Around 20% of cats require tubing replacement at some point. The main risks after surgery are pancreatitis, infection, and occasionally blood clots requiring tissue plasminogen activator flushes to resolve.

Costs for SUB placement in the USA have ranged from approximately US$6,000 to US$15,000 depending on complexity, with additional costs for complications and ongoing monitoring. As with stents, initial estimates should be treated as a minimum rather than a ceiling.

SUB placement is available at a small number of specialist centres globally, including in the USA, Canada, UK, France, Switzerland, and Italy.

Holistic and Complementary Options

Two supplements have been researched in the context of calcium oxalate stones, though neither has been formally studied in cats.

Chanca piedra (Phyllanthus niruri or Phyllanthus amarus) is a tropical plant used in some human and rat studies to inhibit stone formation and increase citrate excretion. One human study found that patients given chanca piedra after lithotripsy had a stone-free rate of 93.5% at six months, compared to 83.3% in those who did not receive it. No equivalent data exist for cats, and vet approval is essential before use.

Renalof is a human dietary supplement derived from activated couch grass reported to reduce stone size in clinical trials in Romania, Indonesia, and Cuba. It is not approved for cats. Do not use without your vet’s guidance.

Prognosis depends on whether stones are static or active, which kidneys are involved, how early treatment begins, and which intervention is available. Many cats with static kidney stones live comfortably for years with no crisis.

When a ureteral obstruction does occur, early intervention gives the best chance of kidney function recovery. Even after successful treatment, some degree of permanent kidney function loss is common. Elevated creatinine levels often persist.

Recurrence of calcium oxalate stones following treatment appears to be relatively low in the short term. One study found only 7% of cats developed new stones within the follow-up period, with a grace period of over two years on average. The true recurrence rate may be higher, and cats who receive interventional treatment require ongoing monitoring.

Prevention is not straightforward, but a number of steps can reduce the risk of new stones forming or existing stones becoming active. Aim for a urine specific gravity below 1.030.

• Feed a canned diet with a water content above 75% to encourage dilute urine. Cats eating exclusively dry food have a higher rate of stone formation.
• Avoid acidifying diets marketed for urinary tract health. These are formulated to prevent struvite and will worsen conditions that favour calcium oxalate.
• Do not give supplements containing calcium, as these can contribute to stone formation.
• If your cat has hypercalcaemia, work with your vet to bring calcium levels under control.
• If a potassium supplement is needed, potassium citrate is the preferred form. Citrate helps alkalinise the urine, reducing the acidic environment in which calcium oxalate stones form. Do not supplement potassium without vet guidance, as some CKD cats have elevated potassium levels.
• If vitamin B6 deficiency is suspected as a contributing factor, a B vitamin supplement may help following vet guidance.
• Thiazide diuretics may be recommended by your vet for cats with frequently recurring stones.
• Keep protein intake around 35% on a dry matter basis, which is appropriate for a CKD cat in any case.
• Have monitoring imaging (x-ray or ultrasound) performed every few months to check whether existing stones have shifted.

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