Introduction

Relative supersaturation (RSS) is a method that measures the potential for urine to dissolve or form crystals, and 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.

Materials and Methods

Three commercial complete dry expanded diets were selected including two diets designed to dissolve struvite uroliths. Those diets were fed successively to 7 cats (4 neutered males, 3 females, 6.0± 2.8 yrs, 5.9 ± 1.3 kg) for 2 weeks. Urinary volume, pH, specific gravity and concentrations of solutes (Ca, Mg, Na, K, NH4+, phosphate, citrate, sulfate, oxalate, uric acid) were measured on the pooled urine for each diet 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 the software SUPERSATTM. For each diet, the pooled urine was allocated to bottles based on mean urine volume produced daily per cat. The bottles were stored at -20°C pending the test. Three groups of feline struvite stones (one group per diet) homogeneous in shape and weight were selected (mean weight: 0.201 ± 0.010 g). Each day and for each diet, a bottle of urine was defrosted, and a group of struvite stones was placed in the urine. The bottle was added to a water bath at 38°C during 24 hours (with a shaking mode for 9 hours to simulate the cat activity). Twenty-four hours later, the urine was filtered to collect the stones. The stones were lightly dried on an absorbent paper and weighed. Then, a new urine bottle was defrosted, and the process restarted until there was complete dissolution of the stones.

Results

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

Discussion

When RSS is < 1 (undersaturation zone), urine dissolves struvite stones efficiently and the lower the RSS, the quicker the dissolution kinetic. RSS is thus a good predictor of the potential of a urine to induce struvite dissolution.