Struvite crystals (magnesium ammonium phosphate)

Struvite crystals  usually appear as colorless, prism-like crystals  or "coffin lids" or old-fashioned double-edged razor blades (lower frame).  Often seen in urine from clinically normal individuals.  Found in urine of any pH, although their formation is favored in alkaline urine.  UTI with urease-positive bacteria can promote struvite crystal formation and urolithiasis by raising urine pH and increasing free ammonia.




Bilirubin crystals 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. 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 common.











Calcium Carbonate

Calcium carbonate crystals usually appear as large yellow-brown or colorless spheroids with radial striations. They can also be seen as smaller crystals with round, ovoid, or dumbbell shapes.


Amorphous crystals

Amorphous crystals appear as aggregates of finely granular material without any defining shape at the light microscopic level.

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.

Calcium oxalate dihydrate crystals sometimes also can present as "amorphous" when the individual crystals are very small. Examination at higher magnification will reveal the typical "envelope" appearance.

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.

Calcium Oxalate Dihydrate

Calcium oxalate dihydrate crystals typically are seen as colorless squares whose corners are connected by intersecting lines. They can occur in urine of any pH. The crystals vary in size from quite large to very small. .

Cystine crystals are flat colorless plates and have a characteristic hexagonal shape with equal or unequal sides. They often aggregate in layers. Their formation is favored in acidic urine.

Cystine crystalluria or urolithiasis is an indication of cystinuria, which is an inborn error of metabolism involving defective renal tubular reabsorption of certain amino acids including cystine. Sex-linked inheritance is suspected since male dogs are almost exclusively affected. Many breeds, as well as mongrels, have been reported affected . Renal function otherwise appears to be normal and, aside from a tendency to form uroliths, the defect is without serious consequence.

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