Evaluation of the VITEK 2 AST-N439 card for susceptibility testing of novel β-lactam/β-lactamase inhibitor combinations and colistin in carbapenem-non-susceptible gram-negative bacilli

We assessed the performance of the VITEK 2 AST-N439 card for susceptibility testing of novel β-lactam/β-lactamase inhibitor (BL/BLI) combinations and colistin in carbapenem-non-susceptible gram-negative isolates. A total of 425 clinical isolates, including Enterobacterales ( n = 242), Pseudomonas aeruginosa ( n = 97), and Acinetobacter baumannii ( n = 86), were tested using VITEK 2, with broth microdilution as the reference standard. Minimum inhibitory concentrations were interpreted according to Clinical and Laboratory Standards Institute breakpoints. The essential agreement (EA), categorical agreement (CA), very major error rate, and major error rate for VITEK 2 were 90.3%, 97.6%, 1.8%, and 2.5% for ceftazidime/avibactam; 81.5%, 89.0%, 7.0%, and 4.7% for ceftolozane/tazobactam; 96.5%, 90.9%, 3.4%, and 1.2% for imipenem/relebactam; and 94.6%, 93.0%, 15.8%, and 3.2% for meropenem/vaborbactam. For colistin, VITEK 2 demonstrated an EA of 89.2% and a CA of 85.4%. Our findings suggest that VITEK 2 has suboptimal performance for novel BL/BLI combinations in carbapenem-non-susceptible gram-negative isolates. Furthermore, despite changes to the colistin formulation used in the VITEK 2 card, it remains an unreliable method for detecting colistin resistance. IMPORTANCE Rapid and accurate antimicrobial susceptibility testing (AST) is essential for managing infections caused by multidrug-resistant gram-negative bacilli. This study evaluated the performance of the VITEK 2 AST-N439 card for susceptibility testing of novel BL/BLI combinations and colistin in carbapenem-non-susceptible gram-negative isolates. Our findings revealed significant limitations, including suboptimal performance for novel BL/BLI combinations and unreliable detection of colistin resistance, even with the updated formulation. These results underscore the need for cautious interpretation of VITEK 2 results and highlight the importance of optimizing its performance to enhance antibiotic decision-making.