Comparison of disk diffusion, MIC test strip, and broth microdilution for susceptibility testing of carbapenem-resistant Enterobacterales with different strains and carbapenemase genotypes to aztreonam/avibactam.

OBJECTIVES: To evaluate the accuracy of MIC Test Strip (MTS, Liofilchem, Italy) and disk diffusion (DD, Liofilchem, Italy) against broth microdilution (BMD) for aztreonam/avibactam (ATM/AVI) susceptibility testing in carbapenem-resistant Enterobacterales (CRE), providing evidence for laboratory method selection and clinical therapy. METHODS: We tested 358 CRE clinical isolates (174 K. pneumoniae, 144 E. coli, 40 other Enterobacterales) by BMD (reference), MTS, and DD, interpreted per Clinical and Laboratory Standards Institute (CLSI) M100 36th Edition and European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoint tables v16.0. Categorical agreement (CA), essential agreement (EA), and error rates were calculated, stratified by carbapenemase genotype (KPC, NDM, KPC+NDM). RESULTS: BMD showed MIC50 of 0.5 mg/L and MIC90 of 2 mg/L (susceptibility rate 98.6%). MTS demonstrated excellent concordance: CA 99.7%, EA 90.2%, and minor error (mE) 0.3%, with 100% CA for KPC-, NDM-, and dual enzyme-producers. DD showed lower CA (90.5% and 91.1% using CLSI and EUCAST breakpoints, respectively) and higher mE (9.5%). Importantly, 9.8% (35/358) of isolates fell within the 22-24 mm intermediate/ATU zone (including 16 blaNDM-5 and 12 blaKPC-2). According to DD, NDM-producers exhibited significantly higher minor error rates than KPC-producers (10.7% vs 5.0%). CONCLUSIONS: MTS is a highly reliable alternative to BMD for ATM/AVI testing across diverse carbapenemase genotypes. While DD is acceptable, the high proportion of isolates in the 22-24 mm zone necessitates mandatory confirmatory testing to ensure precision therapy.