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

## TL;DR

This study evaluates a new automated system for detecting colistin resistance in a type of antibiotic-resistant bacteria, finding it to be mostly accurate compared to traditional methods.

## Contribution

This is the first study to evaluate the cs02n version of the VITEK 2 system for colistin susceptibility in carbapenem-resistant Acinetobacter baumannii.

## Key findings

- The VITEK 2 AST-XN21 system showed 94.6% categorical agreement and 89.1% essential agreement with broth microdilution.
- Very major and major error rates were 3.2% and 2.1%, respectively, with an ISO bias rate of 3.6%.
- No mcr-1 to mcr-5 resistance genes were detected in the 187 isolates tested.

## Abstract

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.

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.