Benchmarking four commercial antimicrobial susceptibility testing systems and disk diffusion to reference broth microdilution in the assessment of gram-negative bacterial isolates

ABSTRACT Commercial antimicrobial susceptibility testing (AST) systems for gram-negative bacteria provide operational advantages over reference broth microdilution (BMD) testing, but the results produced by commercial AST systems might not be as accurate as the reference method. Assessing and comparing the performance of AST systems can be informative for clinical microbiology practice. In total, 280 clinical isolates and 100 isolates from the U.S. Centers for Disease Control and Prevention Antimicrobial Resistance Isolate Bank, including Enterobacterales and Pseudomonas aeruginosa, were intended to be tested using disk diffusion (DD) and automated commercial AST systems, such as Selux AST System (Selux Diagnostics), MicroScan (Beckman Coulter), Phoenix (Becton, Dickinson and Company), and Vitek 2 (bioMérieux). These results were compared to BMD as the reference standard. In total, 372 isolates were assessed. For Enterobacterales, the greatest frequency of very major errors was identified with cefepime by Vitek 2 (29.0%) and ertapenem by DD (9.1%); essential agreement less than 90% was identified with the following combinations: aztreonam by MicroScan (86.4%), aztreonam by Vitek 2 (88.9%), cefepime by Selux (86.3%), cefepime by Vitek 2 (76.8%), ceftazidime by Vitek 2 (89.9%), and levofloxacin by MicroScan (83.4%). For P. aeruginosa , essential agreement less than 90% was identified with the following combinations: gentamicin by Selux (87.2%), tobramycin by Selux (86.6%), and ciprofloxacin by Phoenix (88.4%). Overall, antimicrobials that had low demonstrable bias across all automated commercial AST systems included ampicillin, amoxicillin-clavulanate, ceftazidime-avibactam, ertapenem, and trimethoprim-sulfamethoxazole. Comparing commercial testing systems head-to-head against a reference standard provides insight into the strengths and weaknesses of the performance of these test systems, and these insights can subsequently inform local decisions regarding further investigations and potential operational mitigation strategies. IMPORTANCE This study assessed and compared the performance of multiple commercial antimicrobial susceptibility testing (AST) systems and disk diffusion methods. The accuracy of determining minimum inhibitory concentrations or zone diameters of antimicrobials and the associated interpretive categories were studied for 372 isolates of Enterobacterales and Pseudomonas aeruginosa . Performance of the evaluated systems revealed that multiple antimicrobial-organism combinations were below the standards typically required by regulatory agencies and clinical laboratories. These deficiencies varied across the commercial platforms and disk diffusion methods, highlighting the importance of benchmarking studies that compare the performance using identical isolates. These identified performance weaknesses may warrant further study and/or the implementation of mitigation strategies in clinical laboratory operations to prevent the reporting of errant results.