Factors influencing the overestimation of plasma vancomycin concentrations measured by the Abbott TDx technique

The Abbott TDx technique (TDx) has been reported to overestimate the plasma concentration of vancomycin (VCM) in patients with renal failure and also in those with normal renal function. The aim of this study was to investigate factors influencing the overestimation of plasma VCM concentrations measured by TDx compared with high-performance liquid chromatography (HPLC) as a reference technique. First, the precision and accuracy of TDx and HPLC were compared using 5 weighed-in concentrations of VCM. The coefficients of variation (CV) for both TDx and HPLC were less than 3% at weighed-in concentrations of 3.0, 7.5, 15.0, 30.0, and 60.0 microg/mL. The authors did not find overestimation of VCM concentrations by TDx in any weighed-in sample. Next, VCM concentrations measured by TDx were compared with those measured by HPLC in 253 plasma samples obtained from 83 patients receiving VCM. Regression analysis showed that VCM concentrations measured by TDx were closely correlated with those measured by HPLC, with a correlation coefficient of 0.963. Then, the authors divided plasma samples into 2 groups based on the time of sampling, ie, pre- and postdose. The mean TDx/HPLC ratio was significantly higher in the predose group (1.23 +/- 0.22) than in the postdose group (1.11 +/- 0.16) (P < 0.01). An Eksborg plot showed that the TDx/HPLC ratio decreased slightly with increased total bilirubin concentrations in the predose samples. These results indicated that plasma VCM concentrations measured by TDx were influenced by the time of sampling and the plasma bilirubin concentration. The overestimation of VCM concentrations in the predose group may have resulted from metabolites of VCM because the relative concentrations of the metabolites were likely to be greater in the predose group than in the postdose group. Thus, VCM postdose samples may provide useful information in cases in which a good clinical outcome has not been obtained using monitoring of the predose concentration alone.