Effects of time and dilution on concentration of xanthine in frozen urine and plasma of dogs

Objective - To evaluate the effects of dilution on stability of xanthine in canine urine stored at -20 C, and to evaluate the effects of storage at -20 C on stability of xanthine in canine plasma. Animals - 6 reproductively intact female Beagles, 3.9 to 4.2 years old and weighing 8.5 to 10.1 kg. Procedure - Dogs were fed a 31.4% protein (dry weight), meat-based diet for 21 days, and administered allopurinol (15 mg/kg of body weight, q 12 h) during days 14 to 21; urine and plasma samples were obtained on day 22. Urine samples were preserved undiluted or diluted, and divided into 1-ml aliquots for storage at -20 C for 1 to 12 weeks. Plasma samples were divided into 1-ml aliquots for storage at -20 C for 1 to 12 weeks. Urine and plasma xanthine concentrations were measured on day of collection (baseline) and after 1, 2, 4, 6, 9, and 12 weeks. Results - Dilution of urine samples did not have a significant effect on consistency of xanthine concentration measured for up to 12 weeks of storage. Although xanthine concentration did not differ significantly between undiluted and diluted urine samples, average xanthine concentration measured in diluted samples was consistently higher, compared with that in undiluted samples. Compared with baseline values, plasma xanthine concentration was significantly lower at 6, 9, and 12 weeks of storage. Conclusions - Measurement of xanthine concentration is reproducible in undiluted or diluted urine samples for up to 12 weeks, although dilution may provide better results. Measurement of plasma xanthine concentration is reproducible in samples stored for up to 4 weeks. Clinical Relevance - To ensure reproducibility of measurements of xanthine concentration in urine samples collected from dogs that are affected with urate uroliths and receiving allopurinol, urine should be diluted 1:20 with deionized water. These measurements may be useful for monitoring dogs that are receiving allopurinol for dissolution or prevention of urate uroliths.