Struvite crystals (magnesium ammonium phospha
te, triple phosphate) usually appear as colorless, 3-dimensional, prism-like crystals ("coffin lids"). Occasionally, they instead resemble (vaguely) an old-fashioned double-edged razor blade (lower frame).
Struvite crystals are the most common type in urine from dogs and cats. They are often seen in urine from clinically normal individuals. Though they can be found in urine of any pH, their formation is favored in neutral to alkaline urine.
Urinary tract infection with urease-positive bacteria can promote struvite crystalluria (and urolithiasis) by raising urine pH and increasing free ammonia.
Bilirubin crystals form from conjugated bilirubin (water soluble) and are needle-like to granular crystals that are yellow in color. They tend to precipitate onto other formed elements in the urine. In the top picture, fine needle-like crystals have formed on an underlying cell. This is the most common appearance of bilirubin crystals. In the lower two pictures, cylindrical bilirubin crystals have formed in association with droplets of fat, resulting in a "flashlight" appearance. This form is less commonly seen.
Bilirubin crystals are seen most commonly in canine urine, especially in highly concentrated specimens. They are less common in urine of other species. In dogs, they often are of no clinical significance (healthy dogs can have low, but detectable, bilirubin levels in urine). Bilirubin crystals (or a positive chemical reaction on the urine dipstick) in feline, equine, bovine, or camelid urine is an abnormal finding and the animal should be investigated for an underlying cholestatic process.
Calcium carbonate
Calcium carbonate crystals are variably sized crystals that frequently appear as large spheroids with radial striations. They can also be seen as smaller crystals with round, ovoid, or dumbbell shapes. they are colorless to yellow-brown and can impart a brownish tinge to the urine, when they occur in high numbers.
These crystals are common in the urine of normal horses, rabbits, guinea pigs and goats. They have not been observed in canine or feline urines.
"Amorphous" crystals
"Amorphous" crystals appear as aggregates of finely granular material without any defining shape at the light microscopic level. They can be comprised of urates, phosphates or xanthine. Amorphous urates (Na, K, Mg, or Ca salts) tend to form in acidic urine and may have a yellow or yellow-brown color. Amorphous phosphates are similar in general appearance, but tend to form in alkaline urine and lack color. Xanthine crystals are usually in the form of "amorphous" crystals. These crystals occur in Dalmations on allopurinol therapy for urate urolithiasis.
Generally, no specific clinical interpretation can be made based on the finding of amorphous crystals. Small amorphous crystals can be confused with bacterial cocci in some cases, but can be distinguished by Gram-staining. Degenerating crystals or cells can also resemble "amorphous" crystals.
Calcium oxalate dihydrate crystals
Calcium oxalate dihydrate crystals typically are colorless squares whose corners are connected by intersecting lines (resembling an envelope). They can occur in urine of any pH. The crystals vary in size from quite large to very small. In some cases, large numbers of tiny oxalates may appear as amorphous unless examined at high magnification.
These crystals are often seen in normal urine from domestic animals and can also be an artifact of storage (they can develop in stored urine), emphasizing the need to perform a urinalysis on fresh urine samples.
Urolithiasis due to calcium oxalate has been reported in both dogs and cats. In some cases, they occur secondary to abnormal calcium (increased) excretion due to disorders of calcium metabolism (e.g. hyperparathyroidism). Miniature Schnauzers are predisposed to calcium oxalate urolithiasis, despite no abnormalities in urinary calcium excretion.
Calcium oxalate dihydrate crystals can also be seen in cases of ethylene glycol intoxication, although the picket-shaped form of calcium oxalate monohydrate are pathognumonic. If seen in large numbers in the urine of a dog or cat with acute renal failure and other appropriate clinical signs, consideration should be given to this diagnosis.
Source : © Cornell University
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