How should feline CKD be staged and what does current evidence recommend for management at each stage?

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Feline CKD is staged using fasting serum creatinine and/or symmetric dimethylarginine (SDMA) concentrations measured on at least two occasions in a stable, well-hydrated patient, with substaging added for proteinuria (urine protein:creatinine ratio UPC on at least two samples ≥2 weeks apart) and systolic blood pressure (SBP).36 The International Renal Interest Society (IRIS) framework drives both prognosis and treatment decisions at each stage.13 Dietary modification is the only intervention with robust evidence for improving survival and slowing progression in azotaemic CKD. A protein- and phosphorus-restricted renal diet is recommended from IRIS Stage 2 onward, with use recommended in all Stage 3 and 4 cats.110 Feeding a renal diet to Stage 2 and 3 cats reduces uremic episodes, lowers serum phosphate and fibroblast growth factor 23 (FGF23) concentrations, and increases survival time compared to a conventional diet.10 The primary barrier to dietary management is acceptance: 42% of cats in one prospective evaluation did not successfully transition because either the cat or owner was unwilling to make the change, and compliance is improved by gradual dietary transition.910 For IRIS Stage 1, dietary intervention is currently advised only when FGF23 is elevated, indicating disruption of calcium-phosphorus homeostasis secondary to kidney disease.10 Weight loss and phosphorus dysregulation are present in non-azotaemic cats, and progression to azotaemic CKD can occur over time, making early detection clinically meaningful — but dietary protein and phosphorus restriction in pre-azotaemic cats has not yet demonstrated the same survival benefit as in azotaemic stages.1 Phosphate binders are added when serum phosphate is not controlled by diet alone, or when transition to a low-phosphorus diet has not been achieved. Whether phosphate binders confer the same survival benefit as renal diets is not established.29 Proteinuria and hypertension substaging drives additional pharmaco