Agreement between disk elution in broth and overlay strips for the aztreonam–ceftazidime/avibactam synergy test: variable performance depending on species

Abstract

Background: The disk elution test is a novel technique recently recommended by CLSI to assess the in vitro activity of the synergy between aztreonam (ATM) and ceftazidime/avibactam (CZA). Aim: To compare the agreement between the overlay strip test and disk elution for evaluating synergy between ATM and CZA. Methods: Carbapenem-resistant Gram-negative bacilli with known synergy results from overlay strip testing were selected. Subsequently, disk elution was performed, and the results and agreement were evaluated. Results: Thirty-eight strains were recovered (13 Enterobacterales, 13 Pseudomonas aeruginosa, and 12 Stenotrophomonas maltophilia). 81.6% were producers of metallo-β-lactamases. Positivity using overlay strips and disk elution was 78.9% and 71.0%, respectively, with an overall agreement of 84.2%. In Enterobacterales, positivity was 92% for both methods, with 100% agreement. In S. maltophilia, positivity was 100% for both methods, with 100% agreement. The results for P. aeruginosa were lower: positivity for overlay strips was 46.1%, by disk elution 25.0%, and agreement was 76.9%. Conclusion: Disk elution is a standardized and reproducible technique, demonstrating high activity and agreement in Enterobacterales and S. maltophilia. Performance is low for P. aeruginosa; therefore, its use is not recommended.