Clinical evaluation of EUCAST rapid antimicrobial susceptibility testing in Gram-negative bacilli bloodstream...

Rapid antimicrobial susceptibility testing (RAST) performed directly from positive blood cultures may facilitate earlier optimization of antimicrobial therapy in bloodstream infections. In this single-center retrospective study, we evaluated the clinical performance of the EUCAST RAST method for Gram-negative bacilli bloodstream infections (GNB BSIs) by assessing its agreement with routine susceptibility testing, its potential to support earlier antimicrobial modification, and its effect on turnaround time (TAT) for susceptibility reporting.

A total of 229 patients with Gram-negative bacilli detected in positive blood cultures were collected from November 2025 to March 2026. EUCAST RAST was performed directly from positive blood cultures in parallel with the standard antimicrobial susceptibility testing workflow. RAST categorical agreement (CA) with the comparator method, interpretability of results, potential antibiotic modifications based on RAST, and TAT for susceptibility reporting were evaluated.

A total of 187 isolates (81.7%) met the predefined inclusion criteria and were subsequently included in the final analysis. According to the BD Phoenix M50 system, 1.1% of isolates were carbapenem-resistant and 34.3% were extended-spectrum β-lactamase producers. The overall categorical agreement of RAST for all tested antibiotics was 99.4% at 6 h and 100.0% at 16-20 h, with 70% of results interpretable at 6 h. RAST results supported potential antimicrobial modification in 32.1% of patients, including de-escalation in 7.5% and escalation in 24.6%. Among 16 patients receiving ineffective empirical therapy, 13 (81.3%) could potentially have been switched earlier to active treatment based on RAST results. The median TAT for susceptibility reporting decreased from 44 h (IQR, 39.5-47.0) to 26 h (IQR, 21.5-29.0) compared with short-term methods and from 61.5 h (IQR, 58.0-67.0) to 37.5 h (IQR, 35.0-42.0) compared with conventional methods. The overall mortality rate was only 10.7%.

European Committee on Antimicrobial Susceptibility Testing RAST provided reliable early susceptibility results for Gram-negative bacilli directly from positive blood cultures and substantially shortened reporting time, supporting earlier antimicrobial optimization and antimicrobial stewardship.