Comparison of agar-based methodologies to broth disc elution for the detection of aztreonam susceptibility in the presence of ceftazidime-avibactam.

The combination of aztreonam and ceftazidime-avibactam is a proven treatment option for infections caused by metallo-beta-lactamase (MBL)-expressing gram-negative organisms. Restoration of aztreonam susceptibility in the presence of ceftazidime-avibactam can be determined using the Clinical and Laboratory Standards Institute (CLSI)'s broth disc elution (BDE) method; however, the high manual workload and significant material footprint of this assay make it difficult to implement in a high-throughput clinical microbiology lab. This study evaluates two agar-based methods for the detection of susceptibility to the combination of aztreonam and ceftazidime-avibactam, using BDE as the comparator methodology. Resistant Enterobacterales, Pseudomonas aeruginosa, Stenotrophomonas maltophilia, and Acinetobacter spp. were selected, after which restoration of aztreonam susceptibility was tested using an E-test/disc diffusion (E-DD) method, a double-disc diffusion (DDD) method, and BDE. In comparison to BDE, both E-DD and DDD methods met CLSI M52 validation thresholds with respective categorical agreement rates of 93% and 96%, and no major (MDs) or very major discrepancies (VMDs). In conclusion, agar-based methods such as E-DD or DDD provide alternative methods to evaluate restoration of aztreonam susceptibility in the presence of ceftazidime-avibactam, which can subsequently be used to inform patient care decisions.IMPORTANCEInfections caused by multi-drug-resistant, gram-negative organisms with limited treatment options are associated with significant morbidity and mortality. To avoid patient harm caused by delays in appropriate antimicrobial therapy, the clinical microbiology laboratory must be able to test and report susceptibility to novel antimicrobials or combinations of antimicrobials. The combination of aztreonam plus ceftazidime-avibactam is an empiric, first-line regimen for metallo-beta-lactamase-producing gram-negative organisms that has historically been used without prior susceptibility testing due to a lack of access to gold-standard methods. The Clinical and Laboratory Standards Institute published a broth-based method to test this combination, but it is challenging to implement in a busy clinical laboratory. Two easier-to-implement agar-based methods were successfully validated, which are more accessible and less resource-intensive than the currently described broth-based assays.