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The answer depends on the underlying cause.  Xanthine, an intermediate-byproduct of purine catabolism, is rarely found in urine or stones because it is metabolized to uric acid and then allantoin, a water soluble end-product that is excreted in urine.  Hereditary Xanthinuria is a rare autosomal recessive disease caused by loss of xanthine dehydrogenase (XDH) or molybdenum cofactor sulfurase (MOCOS) genes. Acquired Xanthinuria is a potential adverse consequence of allopurinol, a xanthine dehydrogenase inhibitor administered to treat leishmaniasis and prevent urate uroliths.  In acquired xanthinuria, xanthine uroliths are dissolvable. Hereditary xanthine uroliths are not dissolvable.

Case Example: Jake is a 2-year-old male Dalmatian diagnosed with urate uroliths. Following urolith removal, Jake was prescribed allopurinol (15mg/kg /day) to reduce urate stone recurrence.  However, a diet change was not recommended.  A 2-month recheck identified urolith recurrence (figure 1); uroliths were much larger than his previous ones. During the recheck, small uroliths were retrieved with a urinary catheter and submitted for quantitative analysis.  Recurrent stones were composed of 100% xanthine.  With this new information, allopurinol was discontinued and Jake was prescribed a low purine, urine-alkalinizing diet (Prescription diet u/d).  Radiographs taken one month later revealed that all xanthine stones had dissolved (figure 2).  To prevent future urate urolith recurrence, allopurinol was prescribed at a lower dose (7 mg/kg/day) and the low-purine diet was continued. Read more on allopurinol and urate uroliths.
 Figure 1.  Double contrast cystogram showing xanthine uroliths in the urinary bladder of a male Dalmatian following allopurinol administration.

 

Figure 2. Double contrast cystogram of the dog in figure 1 one month after stopping allopurinol and feeding a low purine diet.  Xanthine uroliths rapidly dissolved.



See Also our Canine Xanthine Recommendations