Chronic kidney disease (CKD) is the most common chronic condition in older cats, affecting an estimated 30 to 40% of cats over 10 years of age and more than 50% of cats over 15. It is a progressive, irreversible loss of functional kidney tissue — specifically the nephrons, the microscopic filtering units that remove waste products, regulate fluid and electrolyte balance, and produce hormones including erythropoietin.
Despite being irreversible, CKD progression can be significantly slowed with appropriate management. The earlier the diagnosis — ideally before clinical signs appear — the more kidney function can be preserved and the better the long-term outcome. Understanding what the key blood biomarkers mean, how they relate to kidney function, and what the staging criteria signify is essential for anyone caring for an older cat.
How the Kidneys Work and Why CKD Matters
Cats have approximately 200,000 nephrons in each kidney. Nephrons filter blood, reabsorb essential nutrients and water, and excrete waste products in urine. CKD results from the progressive loss of nephrons through a combination of ageing, genetic factors, inflammatory disease, infections, toxin exposure, and other insults over the cat’s lifetime.
The kidneys have substantial reserve capacity — clinical signs of CKD do not appear until approximately 66 to 75% of total nephron mass has been lost. This means that by the time a cat is showing symptoms, a significant proportion of kidney function is already gone. Equally importantly, standard blood creatinine levels do not rise above the reference range until approximately 75% of nephron mass is lost. This is the fundamental problem that the SDMA biomarker was developed to address.
| The 75% ProblemA cat can lose up to 75% of its kidney function before creatinine rises above the normal reference range. This means that creatinine alone, without SDMA, will miss CKD until it is already well established. This is why SDMA testing and regular senior screening are essential in cats over 7 years of age. |
Key Biomarkers: SDMA and Creatinine
Creatinine
Creatinine is a metabolic waste product of creatine phosphate breakdown in muscle tissue. It is produced at a relatively constant rate proportional to muscle mass and is excreted almost entirely by glomerular filtration in the kidneys. As kidney function declines and the glomerular filtration rate (GFR) falls, creatinine accumulates in the blood — hence elevated blood creatinine is a marker of reduced kidney function.
The significant limitation of creatinine as a sole CKD marker is its dependence on muscle mass. Cats with low muscle mass (e.g., very elderly, chronically ill, or underweight cats) may have creatinine values within the reference range despite substantially reduced kidney function. Conversely, a muscular cat may have creatinine levels at the top of the normal range or slightly above without true renal disease. Creatinine values should never be interpreted without considering the patient’s body condition score.
SDMA (Symmetric Dimethylarginine)
SDMA is a small amino acid analogue released during the normal breakdown of proteins throughout the body. Like creatinine, it is excreted almost exclusively by glomerular filtration. However, unlike creatinine, SDMA is not significantly affected by muscle mass — its production is proportional to whole-body protein turnover, not just skeletal muscle.
The critical clinical advantage of SDMA is its sensitivity: it rises above the reference range when approximately 25 to 40% of kidney function has been lost — compared to 75% for creatinine. This means SDMA can detect CKD 17 months earlier than creatinine on average, as demonstrated in the landmark SDMA validation studies. It is now included in the IRIS (International Renal Interest Society) CKD staging system as an integral biomarker.
| Parameter | What It Measures | Muscle Mass Effect | Detects CKD At |
| Creatinine | Glomerular filtration (indirectly) | Yes — significant influence | ~75% nephron loss |
| SDMA | Glomerular filtration (indirectly) | No — minimal influence | ~25–40% nephron loss |
| BUN (blood urea nitrogen) | Filtration + protein intake + hydration | Yes — diet and hydration confound | ~75% nephron loss |
| Urine specific gravity (USG) | Concentrating ability of tubules | N/A | Early — before creatinine rises |
| UPC (urine protein:creatinine) | Renal protein loss | N/A | Varies — active disease indicator |
IRIS Staging System for Feline CKD
The International Renal Interest Society (IRIS) staging system is the universally adopted framework for classifying feline CKD severity, guiding management decisions, and providing a shared language for prognosis. Staging is based primarily on fasting plasma creatinine and SDMA levels, assessed on at least two separate occasions in a stable, hydrated cat.
| IRIS Stage | Creatinine (umol/L) | SDMA (ug/dL) | Clinical Description |
| Stage 1 | < 140 | < 18 | Non-azotaemic. Other markers of CKD present (e.g., elevated SDMA, abnormal USG, proteinuria, or imaging changes). Cat typically asymptomatic. |
| Stage 2 | 140–249 | 18–25 | Mild azotaemia. Some cats still asymptomatic; others showing mild PU/PD, mild weight loss. Careful monitoring and dietary management initiated. |
| Stage 3 | 250–439 | 26–38 | Moderate azotaemia. Clinical signs typically present: weight loss, PU/PD, inappetence, poor coat. Management critical to slow progression. |
| Stage 4 | > 440 | > 38 | Severe azotaemia. Uraemic signs: vomiting, oral ulcers, profound lethargy, anorexia. Guarded to poor prognosis; intensive supportive care. |
Each IRIS stage is further sub-staged based on arterial blood pressure (normotensive, pre-hypertensive, hypertensive, severely hypertensive) and proteinuria status (non-proteinuric, borderline, proteinuric), as both hypertension and proteinuria independently accelerate CKD progression and guide additional treatment decisions.
Clinical Signs of CKD
CKD is typically silent in early stages (IRIS 1 and early stage 2). As the disease progresses, the accumulation of uraemic toxins and the loss of renal regulatory functions produce an increasingly broad constellation of signs:
- Polyuria and polydipsia (PU/PD): Often the first owner-noticed sign. Reflects loss of concentrating ability as tubular function declines.
- Weight loss and muscle wasting: Progressive and often dramatic. Results from protein catabolism, reduced appetite, and uraemic cachexia.
- Reduced appetite and nausea: Uraemic toxins stimulate the chemoreceptor trigger zone and cause direct gastric irritation. Anti-nausea medication significantly improves quality of life.
- Lethargy and weakness: Multi-factorial — uraemia, anaemia (reduced erythropoietin production), hypokalaemia, and metabolic acidosis all contribute.
- Oral ulcers and uraemic breath: Uraemic toxins damage oral mucosa. Characteristic ammonia-like (‘uraemic’) odour to the breath in advanced disease.
- Hypertension: Affects approximately 60% of cats with CKD. May cause sudden blindness (hypertensive retinopathy with retinal detachment), neurological signs, or cardiac changes. Blood pressure monitoring is mandatory in all CKD cats.
- Anaemia: Secondary to reduced erythropoietin production and shortened red cell lifespan. Pale mucous membranes, weakness, and reduced exercise tolerance.
Management of Feline CKD
Dietary Management: The Cornerstone
Renal prescription diets are the most evidence-supported intervention for slowing CKD progression. They are formulated to be:
- Phosphorus restricted: Reduced dietary phosphorus slows renal secondary hyperparathyroidism and tubular damage — one of the most important drivers of CKD progression. Studies show significantly longer survival in cats on phosphorus-restricted diets.
- Protein modified: Reduced but high-quality protein reduces the uraemic toxin burden without causing protein malnutrition. The protein reduction is modest in modern renal diets compared to older formulations.
- Increased omega-3 fatty acids: Anti-inflammatory effects on renal tissue. EPA and DHA from fish oil are the most relevant.
Palatability is the limiting factor — a cat that refuses its renal diet and eats nothing receives no benefit. Gradual transition (10 to 14 days minimum), warming the food, and trialling multiple formulations improve acceptance. Phosphate binders (aluminium hydroxide, lanthanum carbonate) can be used when dietary phosphorus restriction alone is insufficient.
Blood Pressure Management
Systemic hypertension (systolic blood pressure consistently above 160 mmHg) requires treatment to prevent end-organ damage — particularly to the eyes, brain, heart, and kidneys themselves. Amlodipine (calcium channel blocker, 0.625 to 1.25 mg per cat q24h) is the first-line antihypertensive for cats and is highly effective. ACE inhibitors (benazepril, enalapril) are used when proteinuria is present, as they reduce glomerular pressure and protein loss.
Fluid Support
Many cats with moderate to advanced CKD benefit from subcutaneous fluid supplementation administered at home by owners. Lactated Ringer’s solution or 0.9% saline given subcutaneously (typically 75 to 150 mL per session, 3 to 7 times per week) supports hydration, reduces uraemic toxin concentrations, and improves quality of life measurably. Many owners find that cats receiving regular subcutaneous fluids are noticeably brighter and more appetent.
Managing Uraemic Signs
- Anti-nausea medications: Maropitant (Cerenia) and ondansetron are effective for uraemic nausea and significantly improve appetite and quality of life.
- H2 blockers / proton pump inhibitors: Famotidine or omeprazole for gastric hyperacidity associated with uraemia.
- Potassium supplementation: Hypokalaemia (low potassium) is common in CKD cats and contributes to weakness and muscle pain. Oral potassium gluconate supplementation is safe and effective.
- Erythropoiesis-stimulating agents: Darbepoetin alfa (subcutaneous injection) can correct anaemia of CKD in cats. Most beneficial in cats with haematocrit below 20%. Monitoring for hypertension and polycythaemia is required.
Monitoring Schedule
| IRIS Stage | Recommended Monitoring Frequency |
| Stage 1 | Every 6 months: blood pressure, SDMA, creatinine, urinalysis, UPC |
| Stage 2 | Every 3–6 months: full profile including electrolytes, phosphorus, packed cell volume, blood pressure |
| Stage 3 | Every 3 months: as above plus assessment of nausea, hydration status, body weight trend |
| Stage 4 | Every 4–8 weeks or as clinically indicated: intensive monitoring; quality of life assessment |
Key Takeaways
- CKD affects over 30% of cats over 10 years old and is the most common chronic condition of older cats
- SDMA detects CKD at 25–40% nephron loss vs. 75% for creatinine — it is the most important advance in early CKD detection
- IRIS staging (Stage 1–4) based on creatinine and SDMA guides management and prognosis
- Phosphorus-restricted renal diets are the single most evidence-supported intervention for slowing CKD progression
- Blood pressure monitoring is mandatory in all CKD cats — hypertension affects 60% and causes severe end-organ damage
- Subcutaneous fluid supplementation at home is practical and significantly improves quality of life in moderate to advanced CKD
References
1. Sparkes AH et al. (2016). ISFM consensus guidelines on the diagnosis and management of feline CKD. J Feline Med Surg 18(3):219–39.
2. Hall JA et al. (2014). Symmetric dimethylarginine: improved functional biomarker for detecting and staging chronic kidney disease in cats. J Vet Intern Med 28(6):1699–714.
3. Ross SJ et al. (2006). Clinical evaluation of dietary modification for treatment of spontaneous CKD in cats. J Am Vet Med Assoc 229(6):949–57.
4. Brown SA et al. (2013). Evaluation of the effects of inhibition of angiotensin converting enzyme with benazepril in cats with induced chronic renal insufficiency. Am J Vet Res 74(3):519–26.
5. Jepson RE (2011). Feline systemic hypertension: classification and pathogenesis. J Feline Med Surg 13(1):25–34.
6. IRIS (2023). IRIS Staging of CKD. International Renal Interest Society. www.iris-kidney.com