Proposed breakpoint of piperacillin/tazobactam against extended spectrum β-lactamases producing bacteria in bacteremia.

The isolation rate of extended-spectrum β-lactamase (ESBL)-producing bacteria have been increasing in Japan. While the efficacy of piperacillin/tazobactam (PIPC/TAZ) for ESBL-producing bacteria is controversial, carbapenems have generally been shown to be effective. The aim of this study was to determine whether the current Clinical and Laboratory Standards Institute susceptibility breakpoint of ≤16/4 μg/mL PIPC/TAZ predicts the clinical usefulness for bacteremia caused by ESBL-producing Enterobacteriaceae. We retrospectively investigated 35 patients with bacteremia caused by Enterobacteriaceae producing ESBLs treated with PIPC/TAZ monotherapy. The microbiological and clinical efficacy with PIPC/TAZ minimum inhibitory concentrations (MICs) of ≤16/4 μg/mL was better than that with MICs ≥ 32/4 μg/mL. In contrast, MICs ≤8/4 μg/mL showed significantly higher microbiological and clinical efficacy compared to that of MICs ≥16/4 μg/mL (P < 0.05). These results suggest that 8/4 μg/mL PIPC/TAZ MIC is recommended as a breakpoint for bacteremia caused by ESBL-producing Enterobacteriaceae in Japan, although the current CLSI breakpoint is also useful.