What they did
Uroliths from 34 dogs and 27 cats were analyzed at the Minnesota Urolith Center. All uroliths were evaluated before exposure to any preservative, and then one urolith from each dog or cat was stored in 1 ml of neutral buffered 10% formalin at room temperature for 48 hours and reevaluated.

What they found
Uroliths examined in the study were composed of:

• Ammonium urate (5 dogs, 5 cats)
• Calcium oxalate (5 dogs, 5 cats)
• Phosphate apatite (5 dogs, 5 cats)
• Cystine (5 dogs, 5 cats)
• Silica (5 dogs)
• Struvite (5 dogs, 5 cats)

A portion of every struvite urolith was transformed into newberyite after exposure to formalin. Newberyite is considered an in vitro artifact and not a biogenic material. Researchers also found that three of the 10 ammonium urate uroliths dissolved when placed in formalin. The mineral composition of all other uroliths was unchanged.

Take-home message
Uroliths should not be exposed to or stored in formalin before quantitative mineral analysis.

Albasan H, Osborne CA, Lulich JP, et al. Effects of storage in formalin on composition of canine and feline uroliths. J Am Vet Med Assoc 2012; 241(12):1613-1616.

Link to abstract: http://avmajournals.avma.org/doi/abs/10.2460/javma.241.12.1613