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By Dr Niquet Reid (BVSc BVMS GPCert (FelP) MANZCVS (Feline Medicine)
Urolithiasis accounts for 7–22% of cases of lower urinary tract disease (LUTD) in cats¹–³ and 18% of LUTD in dogs⁴. Cases involving struvite (magnesium ammonium phosphate) and calcium oxalate (CaOx) stones together account for 80–90% of uroliths⁵–⁷. In-practice diagnostics such as urine pH and sediment analysis offer limited information for risk factors of urolithiasis. Crystals maybe absent despite stone presence (and vice-versa), and urine pH fluctuates throughout the day⁸. Relative Supersaturation (RSS) is a comprehensive methodology considering urine specific gravity, mineral precursors in the urine and urinary pH, to assess the risk of urolith and crystal formation in pets eating a certain food (Figure 1).
Struvite Urolithiasis
Struvite formation is highly pH dependent. RSS studies confirm that acidic urine promotes undersaturation, favouring prevention and dissolution of struvite. Medical or dietary dissolution of struvite stones eliminates the need for surgical removal. In cats, struvite stones are typically sterile and diet responsive. In dogs, they are often associated with urinary tract infections caused by urease-producing bacteria (e.g. Staphylococcus, Proteus), requiring concurrent dietary and antimicrobial therapy.
Calcium Oxalate Urolithiasis
The role of pH in CaOx stone formation is less clear. However, long-term CaOx RSS data does not support a strong correlation with urinary pH⁹. These stones are not amenable to medical dissolution and require removal via surgical or minimally invasive techniques. Prevention focuses on reducing the urinary concentration of stone-forming precursors and promoting dilution, in which diet plays an integral role.
The Role of Urine Dilution
Urine dilution is a cornerstone of urolith prevention. Sodium chloride has been used in prescription diets to effectively promote urine dilution. Studies have shown that such diets do not commonly have adverse effects on blood pressure or renal function in healthy adult and healthy ageing cats¹⁰–¹³. Increased moisture intake also enhances urine volume, reduces retention time, and lowers the risk of crystal formation⁷. Research confirms that moisture-rich diets help reduce urine specific gravity and RSS, particularly in breeds prone to concentrated urine.
Clinical Application
Urine pH and specific gravity (USG) alone offer limited insight due to variability. In cats, a fasted urine pH <6.2 suggests low struvite risk but isn’t definitive, especially with high-moisture diets⁹. USG is a helpful, though imperfect, tool for monitoring recurrence with targets of <1.020 in dogs and <1.030 in cats⁹. RSS remains the most accurate indicator of urolith risk. ROYAL CANIN® Urinary S/O diets lower RSS and help dilute urine, supporting struvite dissolution and preventing both struvite and calcium oxalate stones.
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