The Mechanism of False in Vitro Elevation of Uric Acid Level in Mouse Blood.

Thirty minutes incubation at room temperature elevates the uric acid (UA) level of mouse blood in a test tube, and has previously been reported as "false in vitro elevation of the uric acid level." However the UA level of human blood does not elevate using the same incubation. We clarified the mechanism of the false in vitro UA elevation using mice with highly active hypoxanthine phosphoribosyl transferase (Hprt) of B6-ChrXC(MSM), a consomic mouse strain with the chromosome portion of Mus musculus morocinus in the Hprt gene site, or mice with a targeted deletion of the urate oxidase gene (Uox) (Uox-knockout (KO)). The plasma levels of UA, hypoxanthine, and xanthine, determined by HPLC, were compared with those of C57BL/6J laboratory mice used as controls. The uric acid level of Uox-KO mice was approximately 10 times higher than that of control, did not elevated after incubation in the test tube. With allopurinol, the hypoxanthine levels of B6-ChrXC(MSM) and Uox-KO were significantly lower than that of controls. Without allopurinol, the UA and xanthine levels of B6-ChrXC(MSM) were significantly lower than those of C57BL/6J controls. Even with allopurinol, the UA and xanthine levels were still significantly lower than that of controls. In conclusion, "false in vitro elevation of uric acid level" seems to be caused by low levels of erythrocyte HPRT activity and the low plasma uric acid level of laboratory mice.