Struvite Relative Supersaturation: A Good Predictor of Struvite Stones Dissolution in vitro
ACVIM 2008
C. Tournier; E. Malandain; S. Abouhafs; S. Aladenise; C. Venet; C. Ecochard; R. Sergheraert; V. Biourge
Royal Canin Research Centre
Aimargues, France

Relative supersaturation (RSS) is a method that allows to measure the potential for a urine to dissolve or form crystals and that has been validated in cats. The aim of this study was to assess if struvite RSS is a good predictor of in vitro struvite dissolution kinetic in cat urine.

Two different commercial complete dry expanded diets with 2 levels of Na content, designed to dissolve struvite uroliths were selected: diet A (1.3% Na as fed) and diet B (0.4% Na). Those diets were fed successively to 7 Chartreux cats for 2 weeks. Urinary volume, pH, specific gravity and concentrations of 10 solutes (Ca, Mg, Na, K, NH4+, phosphate, citrate, sulfate, oxalate, uric acid) were measured on the pooled urine collected during the last 7 days of each study period. Based on those data, the urinary relative supersaturation (RSS) for struvite (MAP) was calculated using SupersatTM. Each pooled urine was divided in aliquots based on mean urinary volume produced daily per cats. The aliquots were placed in bottles and stored at -20°C pending the test. Two groups of 2 feline struvite stones (219 ± 1 mg) homogeneous in source, shape and weight were selected. On day 0, a bottle of each urine was defrosted, struvite stones were added. The bottles were then placed in a water bath at 38°C during 24 hours. After that, the urines were filtered to collect the stones. The stones were lightly dried on an absorbent paper and weighed, and the process restarted until complete dissolution of the stones.

During the study, cats remained healthy, maintained their body weight and consumed their diet adequately. Table 1 summarizes the results.

Table 1.


Mean Urine Volume




Number of days for
complete dissolution

Dissolution kinetic

Diet A







Diet B







When RSS is < 1 (undersaturation zone), urine dissolves struvite stones efficiently and the lower the RSS, the quicker the dissolution kinetic. This observation shows that RSS is thus a good predictor of urine potential to dissolve struvite stones. Diet B has also been shown to induce struvite dissolution in vivo. This in vitro model might thus be a good way to assess efficacy in vivo.

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Capucine Tournier

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