Clinical impact of EUCAST rapid antimicrobial susceptibility testing on empirical therapy in bloodstream infections: a real-world observational study

Abstract Purpose This study assessed the impact of EUCAST’s Rapid Antimicrobial Susceptibility Testing (RAST) on empirical therapy and its clinical usability in routine bloodstream infection management. Methods This retrospective single-center, observational study compared two four-month periods before and after RAST implementation (Feb–May 2023 vs. Feb–May 2024) at Oldenburg Medical Center. Patients with monomicrobial bloodstream infections due to Escherichia coli , Klebsiella pneumoniae , Pseudomonas aeruginosa , or Acinetobacter baumannii were included. Primary outcome: duration of empirical antimicrobial therapy. Secondary outcomes: turnaround time and categorical agreement between RAST with VITEK 2, including major errors (ME) and very major errors (VME). Surrogate-based interpretation was used for ATU results, particularly for piperacillin-tazobactam. Results In total 126 patients were included (66 in 2023, 60 in 2024). After RAST, empirical therapy duration was significantly shorter (40.5 h vs. 73.3 h, p < 0.001). Median time to RAST result was 15.8 h, significantly shorter than by VITEK 2 (33.9 h, p < 0.0001), reducing time to first susceptibility result by 9.6 h ( p < 0.00001). Concordance with VITEK 2 was excellent: 100% for cefotaxime, meropenem, and piperacillin-tazobactam, 96.4% for ceftazidime, 95.2% for ampicillin. ATU results occurred mainly for piperacillin-tazobactam and were partly resolved through surrogate-based interpretation. Conclusion RAST significantly shortened empirical therapy duration and accelerated susceptibility reporting with high agreement to VITEK 2. However, antibiotic-specific uncertainty and incomplete translation of early results into therapeutic action indicate that the clinical impact of RAST depends on structured interpretation and integration into antimicrobial stewardship workflows.