When to see a vet urgently
Seek emergency care immediately if swelling or kidney disease is accompanied by any of the following:
- Difficult or open-mouth breathing.
- Collapse or unusual profound weakness.
- A hind leg suddenly becoming painful or unusable.
- Attempts to urinate without passing urine, or rapid deterioration.
You can find a vet near you. New swelling or marked loss of appetite needs prompt examination even without these signs.
Overview
Normally, the glomeruli filter blood without allowing proteins into the urine. When damaged, they allow substantial protein loss (proteinuria). This lowers blood albumin and colloid osmotic pressure, causing tissue swelling. Blood fats rise at the same time, worsening the disease course. In cats, nephrotic syndrome most often complicates chronic glomerular disease and renal amyloidosis.
Albumin is an important blood protein that helps keep fluid within blood vessels. When large amounts are lost in urine, fluid can move into tissues or body cavities. Colloid osmotic pressure describes this effect of blood proteins in retaining fluid within the circulation.
A healthy cat's urine may contain a small amount of protein. The important finding is abnormally heavy, persistent loss whose origin has been established. One protein finding therefore does not mean nephrotic syndrome. Raised blood fats are also part of the assessment, but do not establish severity or prognosis alone.
Causes
- Glomerulonephritis, involving immune-mediated glomerular damage.
- Renal amyloidosis, an abnormal protein accumulation.
- Chronic inflammatory processes, including infections and autoimmune diseases.
- Tumours.
- Toxic kidney damage from medicines or chemicals.
In immune-mediated glomerular disease, immune complexes produced by the defence system can accumulate in the kidney's filtering structures and trigger damage. However, not every glomerular lesion is of this type. Similar protein loss may therefore lead to different treatment decisions in different cats.
Amyloidosis involves accumulation of abnormally folded protein in tissues. In the kidney, its location affects symptoms: amyloid in the glomeruli readily causes marked proteinuria, whereas deposits elsewhere may impair function without comparable protein loss. Not every cat with amyloidosis therefore develops nephrotic syndrome.
Risk factors
- Older age, over seven to eight years.
- Chronic kidney disease.
- Positive FeLV or FIV status.
- Breed predisposition to amyloidosis in Siamese and Abyssinian cats.
- Longstanding inflammatory or infectious diseases.
The age threshold does not apply to every cause. Immune-complex glomerular disease has also been found in young cats, and inherited amyloidosis may appear early. Familial amyloidosis in Siamese cats may predominantly affect the liver, so breed alone does not establish a risk specifically of renal nephrotic syndrome.
Amyloidosis has also been found in non-pedigree rescue cats. Breed or its absence therefore does not rule out the disease. FeLV or FIV findings may form part of the background investigation, but a positive viral test alone does not confirm the type of glomerular damage. Individual investigation is necessary.
Typical symptoms
- Persistent swelling of the abdomen, paws or face.
- Ascites, fluid within the abdominal cavity.
- Lethargy and reduced appetite.
- Weight loss.
- A dull coat and deterioration in general condition.
- Increased urination and thirst with kidney failure.
Kidney filtration and protein retention are different functions. Heavy protein loss may begin before nitrogenous waste products rise clearly in the blood. Visible swelling or low albumin therefore does not require a preceding period of increased drinking and urination.
Abdominal fluid can conceal concurrent muscle loss: weight may not fall as expected despite worsening nutrition. The practice therefore assesses muscles and body condition alongside weight. Report appetite changes, vomiting and whether swelling has increased too.
Complications and consequences
- Chronic kidney failure.
- Blood clots from clotting changes associated with protein loss.
- Severe swelling and breathing problems.
- Multiple organ damage.
- Death without treatment.
Clot risk is taken seriously, but there is less evidence about the size of the protein-loss-related risk in cats than in dogs. The need for medicines affecting clotting is assessed separately. Sudden limb pain or breathing difficulty nevertheless requires immediate care whether or not clot risk was previously recognised.
Blood pressure may also rise and increase the risk of organ damage. Severity is not assessed from swelling alone: kidney function, persistent protein loss and other illnesses also matter.
Diagnosis at the vet
- Urinalysis showing persistent proteinuria.
- Blood biochemistry showing low albumin, raised blood fats and increased nitrogenous waste products.
- Kidney ultrasound for structural changes and amyloid deposits.
- FeLV and FIV testing.
- Kidney biopsy to confirm glomerular disease and amyloidosis.
Ultrasound shows kidney size, structure and other possible abnormalities, but cannot reliably identify amyloid deposits. Confirming amyloidosis requires tissue examination with special staining. Normal-looking kidneys do not rule out glomerular disease either.
The urine protein-to-creatinine ratio (UPC) helps assess protein loss. It is considered alongside urine concentration, cells and other findings. Urinary tract inflammation or bleeding may add protein from sites other than the glomeruli. Bacterial culture and repeat testing are performed when needed to distinguish persistent renal proteinuria from other causes.
Blood samples assess cholesterol, kidney function markers and electrolytes as well as albumin. Blood pressure is measured. Searching for underlying disease may include infection tests and imaging elsewhere in the body. Investigations are chosen according to findings; not every possible infection test is automatically needed for every cat.
A kidney biopsy can distinguish immune-mediated glomerular damage from other changes. It is considered particularly when the information would alter treatment. Bleeding risk is assessed and uncontrolled blood pressure treated beforehand. Biopsy carries risks and is not necessary for every cat with kidney disease; sample handling is planned with the examining laboratory.
Treatment
Treatment aims to reduce protein loss, manage complications and address the identified cause. A seriously ill cat may need hospital care. Removing fluid that hinders breathing may improve comfort but does not repair the kidney filter. Hydration and circulatory status are assessed together.
Proteinuria can be reduced with medicines affecting glomerular pressure regulation. They should not be started in a dehydrated cat or one with circulatory insufficiency without correcting the situation. Blood pressure, potassium and kidney function are monitored during treatment because excessive pressure reduction or other adverse effects may worsen the cat's condition.
Immunosuppressive medication is not automatically appropriate in every case. Its benefit is assessed according to the lesion type and possible infection. Anti-inflammatory medication cannot similarly remove amyloid deposits; amyloidosis management centres on kidney support and control of any inflammation or other underlying factor.
Diet is planned according to disease stage, protein loss and nutritional condition. The vet may recommend a complete renal diet. Do not try to compensate for protein loss by feeding only meat or, conversely, sharply restrict protein yourself. Phosphate and other nutrient needs are assessed from test results.
Home care and prognosis
Provide fresh water and follow the individual food and medication instructions. Do not restrict drinking because of swelling without explicit veterinary advice. Monitor food intake, weight, abdominal size, leg swelling and changes in urination and breathing. Tell the practice if your cat vomits, stops eating or becomes weak.
Check-ups compare urinary protein loss, blood albumin, kidney values and blood pressure with earlier results. Reduced swelling alone does not establish that protein loss has stopped. Medicine effectiveness and tolerability may change, so do not alter dosing at home based on appearance.
Nephrotic syndrome carries a guarded prognosis, but the course depends on the cause and response. Advanced kidney damage and continuing heavy protein loss worsen the outlook. Some underlying factors may be treatable, while irreversible disease shifts the goals towards symptom control and the best possible daily life.
Prevention
- Treat chronic infections and inflammation promptly.
- Treat and monitor immune-related diseases.
- Arrange regular urine and blood tests for older cats.
- Use only medicines safe for the kidneys.
- Keep your cat at a healthy weight and provide balanced nutrition.
Prevention does not guarantee that glomerular disease will not develop. Renal medicine safety is assessed according to the cat's illness, hydration and other medication; no general safety classification identifies a medicine suitable for every cat. Agreed urine and blood checks are useful even while your cat still appears well.
Important for cat owners
Nephrotic syndrome always indicates serious kidney disease. Remember that swelling, ascites or marked lethargy are warning signs. The condition requires an immediate veterinary visit, because early diagnosis and supportive care can slow progression and improve quality of life.
This article provides general information and does not replace veterinary care. Contact a vet if you notice signs of illness in your cat.
Frequently asked questions
Does protein in urine always mean nephrotic syndrome?
No. The syndrome combines significant renal protein loss, low albumin and other characteristic findings. One urine protein result is investigated alongside other sample characteristics and blood results.
Can kidney blood values be normal despite severe protein loss?
Yes. Heavy proteinuria may precede raised nitrogenous waste products. Creatinine alone therefore does not rule out glomerular disease or replace urinalysis.
Does amyloidosis always show on ultrasound?
No. Imaging helps assess kidney structure and other illnesses, but demonstrating amyloid requires tissue examination. The vet weighs whether the biopsy result would provide enough benefit to justify its risks.
This article is for general information only and is not a guide to treatment. If you notice the first signs of illness, contact your vet.
Paw4care editorial team
Sources
- Glomerular disease in dogs and cats, Merck Veterinary Manual, accessed 2026-11-05.
- 2026 iCatCare consensus guidelines on chronic kidney disease in cats, International Cat Care, Journal of Feline Medicine and Surgery, accessed 2026-11-05.
- Treatment recommendations for CKD in cats, 2026, International Renal Interest Society, accessed 2026-11-05.
- Glomerulonephritis in Cats, VCA Animal Hospitals, accessed 2026-11-05.
- Amyloidosis in Cats, VCA Animal Hospitals, accessed 2026-11-05.
- Renal Amyloidosis, Cornell University, accessed 2026-11-05.
- Immune-complex glomerulonephritis in cats: a retrospective study, BMC Veterinary Research, accessed 2026-11-05.
- Renal amyloid-A amyloidosis in cats: Characterisation of proteinuria and biomarker discovery, and associations with kidney histology, Journal of Veterinary Internal Medicine / PubMed, accessed 2026-11-05.
- Initial Triage and Resuscitation of Small Animal Emergency Patients, Merck Veterinary Manual, accessed 2026-11-05.