Comparative evaluation of commercial gradient diffusion and disk diffusion tests for aztreonam–avibactam susceptibility testing

Aztreonam-avibactam (AZA) is a recently approved agent active against metallo-β-lactamase-producing Enterobacterales. However, independent evaluations of commercially available antimicrobial susceptibility testing (AST) products remain limited. We evaluated 56 carbapenem-resistant Enterobacterales isolates using four newly available AZA AST products, including two gradient diffusion tests (bioMérieux Etest; Liofilchem MIC Test Strip) and two disk diffusion tests (Hardy, Liofilchem) across three Mueller-Hinton agar manufacturers at two independent clinical laboratories. A diverse isolate collection enriched for nonsusceptible and breakpoint-adjacent MICs was used to rigorously assess categorical agreement (CA). Broth microdilution (BMD) was used as the reference method. Essential agreement (EA), categorical agreement, error rates, and reproducibility were analyzed. All four commercial AST products demonstrated acceptable analytical performance compared with reference BMD. Gradient diffusion demonstrated high EA (bioMérieux 92.6%, Liofilchem 89.6%), whereas CA ranged from 81.5% to 89.3% across all four products. Minor errors accounted for all categorical discrepancies and occurred almost exclusively among breakpoint-adjacent isolates. Medium-related variability was modest, and quality control (QC) performance was generally high, with strong within-site precision and consistent performance across laboratories. This study provides the first comprehensive head-to-head comparison of commercial AZA susceptibility testing products and supports their implementation in clinical microbiology laboratories.IMPORTANCEAztreonam-avibactam is an important new treatment option for infections caused by carbapenem-resistant Enterobacterales, particularly metallo-β-lactamase-producing organisms, but its clinical use requires reliable susceptibility testing. We provide the first head-to-head evaluation of four newly available commercial gradient and disk diffusion products across two independent laboratories and three Mueller-Hinton agar manufacturers. By using a diverse isolate collection enriched for MICs near clinical breakpoints, we also subjected these products to a stringent assessment of categorical agreement. Gradient diffusion demonstrated high essential agreement, and no major or very major errors remained after repeat and discrepancy testing with any product. These findings provide timely, practical evidence to guide clinical laboratories implementing aztreonam-avibactam susceptibility testing.