Response to the comments of K. Day and J. Zakowski on "Clinical capillary zone electrophoresis of serum proteins: balancing high sensitivity and high specificity".

vitro uricolytic activity of Rasburicase, and we undertook two experiments. We retrieved a sample from day 8 of admission [pre-Rasburicase plasma uric acid concentration, 1.59 mmol/L (270 mg/L)] and another from day 14 of admission [post-Rasburicase plasma uric acid concentration, 0.05 mmol/L ( 8 mg/L)]. These two samples had been stored at 4 °C. In the first experiment, we mixed the day 8 and day 14 samples in equal proportions (0.5 mL:0.5 mL). This sample was kept at room temperature ( 25 °C), and uric acid analysis was performed at 30min intervals for 3 h. As seen in Fig. 1 (RT1), there was a rapid decrease in uric acid concentrations within 3 h, confirming ongoing in vitro enzymatic degradation of uric acid. In the second experiment, we allowed one aliquot of the mixed sample (as above) to stand at room temperature and kept another at 4 °C. This experiment was carried out on day 26, but the two samples were taken on days 8 and 14, respectively. In vitro enzymatic uric acid degradation in the mixed samples continued at room temperature (RT2). The uric acid concentrations remained relatively stable in the sample maintained at 4 °C. The change from baseline to 6 h was 0.05 mmol/L (8 mg/L) and was not statistically significant. This finding demonstrates that the in vitro uricolytic activity of Rasburicase is minimized in a sample maintained at 4 °C as reported previously (1, 3). We conclude that, in monitoring uric acid concentrations in patients receiving recombinant uric acid oxidase therapy, blood samples should be kept on ice immediately after collection, during specimen transport, and before analysis. As long as compliance of sample handling is achieved at 4 °C, the results of uric acid measurements should be valid.