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Being crystal clear about crystalluria
Crystals only form in urine that is supersaturated with lithogenic substances. Therefore, crystalluria represents a risk factor for urolith formation. Crystalluria can aid prediction of urolith composition, serve as a marker for undiagnosed disease (e.g. urate crystals in dogs with protovascular shunts), and index therapy effectiveness (e.g. castration to resolve cystinuria).
![calcium oxalate mono and dihydrate crystals in urine sediment](/sites/vetmed.umn.edu/files/styles/folwell_third/public/2022-11/com_cod.png?itok=u4WF8XL4)
Crystalluria (or lack thereof) should never be used as the sole indicator for urolith disease. Here are reasons why:
- Uroliths can be present without crystals.
- Crystals can be present without uroliths.
- Increases in urine pH favor struvite crystals even in dogs and cats with non‐struvite stones.
- Once uroliths form, crystalluria lessens in severity or disappears; this is because less energy is needed to attach minerals to preformed uroliths (stone growth) than to precipitate to form new crystals.
![struvite crystals in urine](/sites/vetmed.umn.edu/files/styles/folwell_third/public/2022-11/map_crystals.png?itok=vrgykdrG)
Although crystalluria is proof that urine is supersaturated with lithogenic substances and a risk for urolith formation, care must be used to not over‐interpret the significance of crystalluria in urine samples that are:
- collected by owners
- refrigerated prior to analysis
- sent out to laboratories for analysis
- analyzed more than 30 minutes to 2 hours after collection
![cystine urine crystals in a dog as seen from a microscope](/sites/vetmed.umn.edu/files/styles/folwell_third/public/2022-11/cystine_crystals.png?itok=owK7CyRa)
In vitro changes may not represent in vivo conditions. Therefore, analyze urine in your clinic as soon as feasible after collection to achieve the most reliable results.