Effects of storage in formalin on composition of canine and feline uroliths

Abstract Objective—To determine whether storage in neutral-buffered 10% formalin in vitro has any effect on the composition of biogenic minerals of canine and feline uroliths. Design—Prospective in vitro study. Sample Population—Canine and feline uroliths submitted to the Minnesota Urolith Center from 34 dogs and 27 cats. Procedures—Submissions from each dog or cat consisted of multiple uroliths of a single mineral type. After retrieval from the urinary tract, none of the uroliths had been placed in a preservative before submission. Evaluated uroliths were exclusively composed of the following: only struvite (uroliths from 5 dogs and 5 cats), calcium oxalate (5 dogs and 5 cats), calcium phosphate apatite (5 dogs and 5 cats), cystine (5 dogs and 5 cats), ammonium urate (5 dogs and 5 cats), or silica (5 dogs). One urolith from each dog or cat was quantitatively analyzed by polarized light microscopy, infrared spectroscopy, or both. Another urolith from the same animal was immersed in 1 mL of neutral-buffered 10% formalin for 48 hours at room temperature (22.5°C). Uroliths exposed to formalin were then air-dried for 30 minutes, and the analysis was repeated. Results—After exposure to formalin, a portion of every struvite urolith was transformed into newberyite. This was not observed with any other urolith mineral type. Quantitative mineral analysis of nonstruvite uroliths revealed no detectable change in mineral composition. However, 3 of 10 ammonium urate uroliths dissolved when placed in formalin. Conclusions and Clinical Relevance—To avoid misdiagnosis of mineral composition, uroliths should not be immersed in formalin prior to analysis.