Objectives: To determine the β-lactam exposure associated with positive clinical outcomes for Gram-negative blood stream infection (BSI) in critically ill patients. Patients and methods: Pooled data of critically ill patients with mono-microbial Gram-negative BSI treated with β-lactams were collected from two databases. Free minimum concentrations (fCmin) of aztreonam, cefepime, ceftazidime, ceftriaxone, piperacillin (co-administered with tazobactam) and meropenem were interpreted in relation to the measured MIC for targeted bacteria (fCmin/MIC). A positive clinical outcome was defined as completion of the treatment course or de-escalation, without other change of antibiotic therapy, and with no additional antibiotics commenced within 48 h of cessation. Drug exposure breakpoints associated with positive clinical outcome were determined by classification and regression tree (CART) analysis. Results: Data from 98 patients were included. Meropenem (46.9%) and piperacillin/tazobactam (36.7%) were the most commonly prescribed antibiotics. The most common pathogens were Escherichia coli (28.6%), Pseudomonas aeruginosa (19.4%) and Klebsiella pneumoniae (13.3%). In all patients, 87.8% and 71.4% achieved fCmin/MIC ≥1 and fCmin/MIC >5, respectively. Seventy-eight patients (79.6%) achieved positive clinical outcome. Two drug exposure breakpoints were identified: fCmin/MIC >1.3 for all β-lactams (predicted difference in positive outcome 84.5% versus 15.5%, P < 0.05) and fCmin/MIC >4.95 for meropenem, aztreonam or ceftriaxone (predicted difference in positive outcome 97.7% versus 2.3%, P < 0.05). Conclusions: A β-lactam fCmin/MIC >1.3 was a significant predictor of a positive clinical outcome in critically ill patients with Gram-negative BSI and could be considered an antibiotic dosing target. © 2019 The Author(s) 2019. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For permissions, please email:
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