Evaluation of an automated antimicrobial susceptibility testing system performance for colistin susceptibility in carbapenem-resistant Acinetobacter baumannii isolates.

This study aims to assess the efficacy of VITEK 2 AST-XN21 (bioMérieux, Marcy l'Étoile, France) cards in identifying colistin susceptibility, comparing their performance with broth microdilution (BMD) results. A total of 187 carbapenem-resistant Acinetobacter baumannii were included and identified by MALDI-TOF (VITEK MS, bioMérieux, Marcy l'Étoile, France), and colistin susceptibility was determined by BMD and the VITEK 2 AST-XN21 card. The cs02n version is used in the VITEK 2. Isolates were evaluated for essential (EA) and categorical (CA) agreements, error rates, and ISO 20776-2:2021 bias rate (acceptable if bias rate is between -30% ≤ bias ≤+30%). Minimum inhibitory concentration (MIC) was interpreted according to the European Committee on Antimicrobial Susceptibility Testing breakpoints (susceptible ≤2 µg/mL; resistant >2 µg/mL). All isolates were subjected to in-house multiplex PCR for possible resistance genes mcr-1, mcr-2, mcr-3, mcr-4, and mcr-5. Out of 187 isolates, 16.04% (n = 30) of all isolates were determined as colistin resistant by the BMD method. MIC₅₀ and MIC₉₀ of the isolates were detected as 1 and 4 mg/L, respectively. When VITEK 2 AST-XN21 results were compared with BMD, CA and EA were 94.6% (177/187) and 89.1% (156/175), respectively. Very major error (3.2%, 6/187) and major error (2.1%, 4/187) were detected. ISO 20776-2:2021 bias rate was 3.6%. No mcr1-5 genes were detected.

Importance: Colistin resistance in carbapenem-resistant Acinetobacter baumannii (CR-Ab) remains a global problem, and broth microdilution constitutes the workload of routine clinical microbiology laboratories. This study is the first to evaluate the cs02n version in the VITEK 2. Our results suggest that VITEK 2 AST-XN21 cards could be an alternative method to detect colistin resistance in CR-Ab isolates.