Different methods are used to determine the minimum inhibitory concentration (MIC) for daptomycin. The threshold is unknown for the free drug area under the concentration–time curve to MIC ratio ( f AUC/MIC) of daptomycin using broth microdilution (BMD) to predict outcome of vancomycin-resistant enterococcus (VRE) bacteremia. The MIC testing method which is best for predicting the outcome remains unclear.

This is a retrospective cohort study. The inclusion criterion was VRE bacteremia treated with ≥ 8 mg/kg of daptomycin. As we aimed to compare different daptomycin MIC testing methods for predicting the clinical outcome of VRE bacteremia, the inclusion criteria included the availability of MIC values for BMD, Etest, and automated antimicrobial susceptibility testing (AST). The primary end point was 28-day mortality. The f AUC/MIC was dichotomized using classification and regression tree analysis for predicting survival.

A total of 393 patients were included; 215 survived and 178 died. In the multivariable logistic model for predicting mortality, the dichotomized f AUC/MICs for Etest and AST were 0.508 and 0.065 times as probable, respectively, as that for BMD to minimize information loss. An f AUC/MIC >  75.07 for BMD significantly predicted lower mortality (adjusted odds ratio, 0.53, 95% confidence interval, 0.30–0.95; P  = 0.03) after adjusting for underlying disease and bacteremia severity. Using Monte Carlo simulation, none of the doses had a probability of target attainment of ≥ 50% with an MIC of ≥ 2 mg/L.

The dichotomized threshold for f AUC/MIC for BMD was the best predictor of mortality. An f AUC/MIC >  75.07 for BMD independently predicted better survival.