In the Belgian national external quality assessment scheme, we observed significantly lowered recoveries for urate in the group of Beckman Synchron(R) users in comparison with the overall median values of the uricase/peroxidase colorimetric methods. These effects were linked to control sera of some manufacturers and we could demonstrate that these sera contained large amounts of pyridoxal-5-phosphate (PLP) as activator for transaminases. By spiking normal human serum with increasing PLP concentrations from 62.5 to 1000 mu moll(-1) we observed a decrease in the urate recovery from 125 mu moll(-1) (-11%). At 1000 mu moll(-1) PLP, only 40% of the urate concentration was measured. An explanation for this effect was found in the polychromatic corrections of the Beckman Sychron(R) system only applied with the Beckman urate method. This study demonstrates that EQAS organizers must carefully distinguish in their peer groups, not only the analytical principle and the measurement equipment, but also the reagent origin. Finally, the use of EQA control sera without PLP addition is strongly recommended if these sera are intended to be used as accuracy controls in EQA schemes including Beckman Synchron(R) users.
