INSTABILITY OF STANDARD CALIBRATORS MAY BE INVOLVED IN OVERESTIMATINGVANCOMYCIN CONCENTRATIONS DETERMINED BY FLUORESCENCE POLARIZATION IMMUNOASSAY

Fluorescence polarization immunoassay (FPIA) is widely used to determi ne serum vancomycin concentrations, and it has been shown to overestim ate vancomycin concentrations in sera from renally impaired patients. This phenomenon has generally been thought to result from interference by vancomycin crystalline degradation products (CDP-1). In this study , we confirmed that serum vancomycin concentrations in various patient s determined by FPIA were higher than those determined by high-perform ance liquid chromatography (HPLC) or enzyme multiplied immunoassay (EM IT). However, the quantitative differences in the serum vancomycin con centrations determined by FPIA versus HPLC were higher than the CDP-1 concentrations, even when the cross-reactivity of FPIA to CDP-1 is ass umed to be 100%. When the vancomycin calibrators for FPIA were stored at 4 degrees C for 30 days, their concentrations determined by FPIA an d HPLC decreased by 14 and 20%, respectively, and CDP-1 corresponding to 20% of primary vancomycin was formed. When stored at 25 degrees C, the degradation of vancomycin was more marked. We concluded that not o nly the cross-reactivity of FPIA to CDP-1 but also the instability of calibrators may cause the overestimation of serum vancomycin concentra tions determined by FPIA.