A multicenter performance evaluation of aztreonam-avibactam gradient diffusion susceptibility testing for Enterobacterales

Aztreonam-avibactam (AZA) is a new monobactam/beta-lactamase inhibitor combination that recently received regulatory approval in the United States (U.S.) and Europe. In this study, we evaluated the performance of ETEST AZA, a gradient diffusion method, compared to reference broth microdilution for 650 challenge (n = 77) and clinical (n = 573) Enterobacterales isolates across four study sites. Using the U.S. Food and Drug Administration (FDA) interpretive breakpoints for ETEST AZA, challenge isolates demonstrated a 97.3% (72/74) essential agreement (EA), 93.2% (69/74) categorical agreement (CA), 0% very major errors (VME) and major errors (ME), and 6.8% (5/74) minor errors (mE). Clinical isolates demonstrated a 95.5% (504/528) EA, 98.7% (521/528) CA, 0% VME, 0.2% (1/516) ME, and 1.1% (6/528) mE. Overall, the combined set achieved a 95.7% (576/602) EA and 98.0% (590/602) CA, with 0% VME, 0.2% (1/577) ME, and 1.8% (11/602) mE. Using the European Committee on Antimicrobial Susceptibility Testing (EUCAST) interpretive breakpoints, challenge isolates demonstrated a 96.1% (74/77) EA, 93.5% (72/77) CA, and 31.3% (5/16) VMEs, while clinical isolates demonstrated a 94.1% (539/573) EA and 99.7% (571/573) CA including 7.7% (1/13) VME and 0.2% (1/560) ME. The combined set demonstrated a 94.3% (613/650) EA, 98.9% (643/650) CA, 0.2% (1/573) MEs, but 20.7% (6/21) VMEs due to the lack of an intermediate breakpoint that is addressed as a limitation in the European package insert. Both bias and trending were in alignment with regulatory requirements. In reproducibility testing of 10 on-scale strains, 100% (270/270) of the results were within ±1 doubling dilution of the modal minimal inhibitory concentration. In conclusion, ETEST AZA provides accurate, reproducible susceptibility results that meet regulatory requirements for many Enterobacterales species when interpreted using FDA or EUCAST criteria. Importance: Aztreonam-avibactam (AZA) is a novel monobactam/β-lactamase inhibitor combination approved for clinical use in the United States and Europe in 2025 and 2024, respectively. It is indicated for the treatment of multidrug-resistant Gram-negative bacteria that produce multiple classes of β-lactamases. Until recently, antimicrobial susceptibility testing for this combined agent relied on broth disk elution or microdilution using ceftazidime-avibactam and aztreonam combined. In this study, we evaluated the performance of the bioMérieux ETEST AZA gradient diffusion strip as a more streamlined method for clinical microbiology laboratories. Testing of 650 Enterobacterales isolates demonstrated that ETEST AZA met regulatory performance criteria using interpretive breakpoints from both the FDA and the European Committee on Antimicrobial Susceptibility Testing. Overall, our findings indicate that the ETEST AZA demonstrates acceptable performance for many Enterobacterales isolates and represents a practical alternative for implementing AZA susceptibility testing in clinical microbiology laboratories.