Impact of the MIC of piperacillin/tazobactam on the outcome for patients with bacteraemia due to Enterobacteriaceae: the Bacteraemia-MIC project

Our objective was to evaluate the impact of low versus borderline MIC of piperacillin/tazobactam on the clinical outcomes of patients with bacteraemia caused by Enterobacteriaceae who were treated with that antimicrobial. A prospective observational multicentre cohort study was conducted in 13 Spanish university hospitals. Patients > 17 years old with bacteraemia due to Enterobacteriaceae who received empirical piperacillin/tazobactam treatment for at least 48 h were included. Outcome variables were clinical response at day 21, clinical response at end of treatment with piperacillin/tazobactam and all-cause 30 day mortality. Univariate and multivariate logistic regression analyses were performed. Overall, 275 patients were included in the analysis; 248 (90.2%) in the low MIC group (a parts per thousand currency sign4 mg/L) and 27 (9.8%) in the borderline MIC group (8-16 mg/L). The biliary tract was the most common source of infection (48.4%) and Escherichia coli was the most frequent pathogen (63.3%). Crude 30 day mortality rates were 10.5% and 11.1% for the low MIC group and the borderline MIC group, respectively (relative riskaEuroS=aEuroS1.06, 95% CIaEuroS=aEuroS0.34-3.27, PaEuroS=aEuroS1). Multivariate analysis of failure at day 21 and at end of treatment with piperacillin/tazobactam and 30 day mortality showed no trend towards increased clinical failure or mortality with borderline MICs (ORaEuroS=aEuroS0.96, 95% CIaEuroS=aEuroS0.18-4.88, PaEuroS=aEuroS0.96; ORaEuroS=aEuroS0.47, 95% CIaEuroS=aEuroS0.10-2.26, PaEuroS=aEuroS0.35; ORaEuroS=aEuroS1.48, 95% CIaEuroS=aEuroS0.33-6.68, PaEuroS=aEuroS0.6). We did not find that higher piperacillin/tazobactam MIC within the susceptible or intermediate susceptibility range had a significant influence on the outcome for patients with bacteraemia due to Enterobacteriaceae.