Comparative evaluation of EUCAST RAST and QuickMIC for rapid antimicrobial susceptibility testing of carbapenem-resistant organisms directly from positive blood cultures

ABSTRACT Rapid availability of phenotypic susceptibility results is crucial for the timely detection of multidrug-resistant Gram-negative organisms. Several novel methods have been introduced that enable direct antimicrobial susceptibility testing (AST) from positive blood cultures. This study aimed to assess the analytical performance of the agar diffusion-based EUCAST rapid AST (RAST) method and the automated QuickMIC system using a challenging collection of highly resistant Gram-negative organisms. One hundred one Gram-negative bacteria ( Escherichia coli , n = 24; Klebsiella pneumoniae , n = 22; Acinetobacter baumannii , n = 30; and Pseudomonas aeruginosa , n = 25) were spiked into blood cultures and processed according to the respective AST protocols. Broth microdilution (BMD) was performed as the reference method. Time to result (TTR), categorical agreement (CA), and essential agreement (EA) with BMD were evaluated. Boruta analysis was applied to identify genetic determinants associated with AST errors. Overall TTR for QuickMIC was 3 h 44 min with a CA of 86.2% and an EA of 92.3% for Enterobacteriaceae and 97.0% for non-fermenters. Overall CA of RAST ranged from 90.7% to 93.7% across reading time points. Overall, very major discrepancy rates were low (QuickMIC, n = 0.7%, and RAST, n = 0.1%). Presence of NDM-5 and KPC was most frequently associated with errors for QuickMIC and EUCAST RAST, respectively. Both rapid AST approaches yielded rapid and robust results in this diverse and highly resistant bacterial study population. These findings underscore the potential of rapid AST methods to facilitate timely optimization of antimicrobial therapy in bloodstream infections, even in the context of extensively drug-resistant pathogens. IMPORTANCE Accurate antimicrobial susceptibility testing (AST) is essential for stewardship and effective therapy, especially as rising antimicrobial resistance increases the risk of empiric treatment failure. Traditional AST methods are limited by slow turnaround times, creating a need for rapid alternatives. This study evaluated the diagnostic accuracy of two rapid AST (RAST) methods—EUCAST RAST and QuickMIC—using 101 genetically characterized, carbapenem-resistant Enterobacterales, Pseudomonas aeruginosa , and Acinetobacter baumannii tested directly from positive blood cultures. Broth microdilution served as the reference. Both rapid assays provided results within 3.5–6 h and demonstrated high categorical and essential agreement with few very major discrepancies. Incorrect results were more common in isolates harboring NDM-5 and KPC carbapenemases. Overall, the findings support EUCAST RAST and QuickMIC as reliable tools for challenging resistant pathogens and highlight their potential to enable earlier detection of carbapenem-resistant phenotypes and more timely initiation of appropriate, last-resort antimicrobial therapy.