Evaluation of Microscan WalkAway for determination of ceftazidime/avibactam and ceftolozane/tazobactam susceptibility in carbapenem-resistant Gram-negative bacilli.

Background: We evaluated the performance of ceftazidime/avibactam and ceftolozane/tazobactam MicroScan Neg multidrug-resistant MIC 1 NMR1 panel for clinical carbapenem-nonsusceptible Gram-negative bacilli isolates. Methods: We evaluated 212 clinically significant carbapenem-nonsusceptible Gram-negative bacilli 139 Pseudomonas aeruginosa and 73 KPC-producing Enterobacterales from 71 Brazilian hospitals 2013-2020. Ceftazidime/avibactam and ceftolozane/tazobactam MICs from the panel were compared with broth microdilution BMD test as the reference method. Essential agreement EA and categorical agreement CA were assessed. For P. aeruginosa, antimicrobial susceptibility testing error rates were calculated using the error-rate bound method. Results: Discrepancies were initially observed with 11 isolates, 4 resolved after retesting, 2 in favor of the NMR1 and 2 in favor of the BMD method. The ceftazidime/avibactam EA overall and evaluable was 100% for P. aeruginosa and Enterobacterales. CA was 100% for Enterobacterales, and 98.6% for P. aeruginosa. The ceftolozane/tazobactam EA was 98.6% and 100% overall and evaluable, respectively, and CA was 96.4% for P. aeruginosa. For ceftazidime/avibactam, no VME was found, and the ME rate was 4.2% 2/48. For ceftolozane/tazobactam and P. aeruginosa, using the CLSI breakpoints, the minor error mE was 11.4% and no VME or ME was found. While using EUCAST breakpoints, VME was 11.4% with no ME. The mE becomes ME or VME in the absence of the intermediate category. All categorical errors were also within 1 log of MIC variation, and the adjusted error rate for CLSI/EUCAST was 0% 0/212. Conclusions: The NMR1 panel is an option to test ceftazidime-avibactam for KPC-producing Enterobacterales and carbapenem-nonsusceptible P. aeruginosa. When ceftolozane-tazobactam MIC 4 mg/L are obtained using this method, an alert could be created, and the results could be confirmed by alternative method.