Effect of urine storage on urinary uric acid concentrations

Accurate determination of serum and urinary uric acid concentrations is ess ential for the diagnosis and classification of gout according to uric acid metabolism derangement. Urine and/or serum samples are often kept at either 4 degrees C or -20 degrees C until assayed, when a large number of samples are handled simultaneously. Our preliminary study indicated a significant decrease in urinary uric acid concentration after preservation, regardless of the storage temperature. Uric acid crystals were often observed in these cases which showed a marked decrease in urinary uric acid concentration af ter storage. In the present study, we sought the factor(s) that might cause this decrease in urinary uric acid concentration, as well as measures to o vercome the problem. High urinary uric acid concentration and low pH proved to play major roles in the decrease in urinary uric acid concentration aft er storage. In contrast, dilution of the urine samples before storage resul ted in no significant change in urinary uric acid concentration. Based on t hese results, we recommend diluting urine before storage for determination of uric acid concentration and avoiding underestimation.