Effects of various methods of preservation on the stability of uric acid in frozen canine urine

Objective - To evaluate the effects of dilution and alkalinization, separately and together, on the stability of uric acid in canine urine stored at -20 C. Design - Prospective-controlled study. Animals - 5 dogs with confirmed ammonium urate uroliths, 6 Beagles, and 6 mixed-breed dogs. Procedure - Dogs were fed a 31.4% protein (dry weight), meat-based diet for 21 days, and urine samples were collected on day 22. Urine samples were preserved, using combinations of dilution and alkalinization, and divided into 1-ml aliquots for storage at -20 C for 1 to 12 weeks. Urine uric acid concentrations were measured, using high-performance liquid chromatography, on day of collection (baseline), and after 1, 2, 4, 8, and 12 weeks. Results - Alkalinization did not have a significant effect on reproducibility of measurements of uric acid concentrations in urine; however, dilution did have a significant effect. Compared with baseline, uric acid concentrations in urine samples collected from dogs with ammonium urate uroliths and Beagles and diluted 1:10 or 1:20 with deionized water were not different after storage for 1 to 12 weeks. Uric acid concentrations in urine samples collected from mixed-breed dogs did not differ from base-line values during the 12-week storage period whether samples were undiluted or were diluted 1:10 or 1:20 with deionized water. Conclusions - Measurements of uric acid concentration are most reproducible in canine urine samples stored at -20 C for 1 to 12 weeks when samples are diluted 1:20 with deionized water. Clinical Relevance - To ensure reproducibility of measurements of uric acid concentration in urine samples collected from dogs affected with urate uroliths, urine should be diluted 1:20 with deionized water. Alkalinization is not necessary, and is not recommended because of the additional step in processing and its potential to interfere with measurement of other urinary analytes.