Comparison of antimicrobial susceptibility testing methods for aztreonam-avibactam against NDM-producing Enterobacterales clinical isolates

Aztreonam-avibactam (ATM-AVI) is a promising therapeutic for New Delhi metallo-β-lactamase-producing Enterobacterales (NDM-E) infections. We evaluated the performance of disk diffusion and gradient diffusion methods compared with reference broth microdilution (BMD) for ATM-AVI susceptibility testing. A total of 245 non-duplicate NDM-E isolates were included, collected from unique patients at hospitals in Chile and the United States between 2022 and 2024. Nine ATM-AVI non-susceptible isolates from a separate collection were added to enrich for resistant isolates, yielding a total of 254 isolates. Reference BMD was performed according to Clinical and Laboratory Standards Institute (CLSI) criteria. ATM-AVI disk diffusion and two gradient diffusion methods (ETEST and MIC Test Strip) were evaluated against BMD using categorical agreement, essential agreement, and CLSI-defined error rates. All 245 NDM-E isolates were susceptible to ATM-AVI (MIC ≤4/4 µg/mL), with MIC₅₀/₉₀ values of 0.25/4 and 1/4 µg/mL, respectively. The collection consisted of Klebsiella pneumoniae (71%), Enterobacter cloacae complex (21%), and Escherichia coli (8%). Predominant NDM alleles included bla NDM-1 and bla NDM-7 . Disk diffusion demonstrated 98% categorical agreement (250/254), with four minor errors. Both gradient diffusion methods achieved 99% categorical agreement (252/254), with no very major or major errors and two minor errors. Essential agreement was 67% for ETEST and 85% for MIC Test Strip; both methods tended to underestimate ATM-AVI MICs relative to BMD. ATM-AVI demonstrated robust in vitro activity against a contemporary NDM-E cohort. Disk and gradient diffusion methods are practical alternatives to BMD for clinical laboratories.