Assessment of colistin resistance in Enterobacteriaceae: Comparison of the rapid polymyxin NP test, MicroScan system, and broth microdilution in Thailand

Background: Antimicrobial resistance is a global health crisis, with colistin serving as a last-resort antibiotic for the treatment of multidrug-resistant Enterobacteriaceae. However, colistin resistance is becoming increasingly prevalent, necessitating rapid and accurate susceptibility testing. Objective: This study evaluated the rapid polymyxin NP (RPNP) test and the MicroScan antimicrobial susceptibility system compared to the reference broth microdilution (rBMD) method for detecting colistin resistance in colistin-resistant Enterobacteriaceae (CRE) isolates from clinical setting in Thailand. Materials and methods: A total of 158 non-repetitive Enterobacteriaceae isolates (85 Escherichia coli and 73 Klebsiella pneumoniae) were collected from a tertiary hospital in Bangkok between August 2021 and January 2023. Colistin susceptibility was assessed using the RPNP, MicroScan system, and rBMD method following the Clinical and Laboratory Standards Institute (CLSI) guidelines. The categorical agreement (CA), very major errors (VME), and major errors (ME) of colistin resistance test results from the RPNP and MicroScan methods were determined by comparison with the rBMD method. Results: Among the 158 Enterobacteriaceae isolates, 46.2% were intermediate and 38.6% were colistin-resistant by rBMD. The sensitivity of RPNP was 98.4% and the specificity was 95.9%, compared to 95.1% and 97.9% for MicroScan. Both RPNP and MicroScan achieved high CA (96.8%) with rBMD. Discrepancies, primarily in isolates with borderline MICs (1-4 mg/L), accounted for 4 of the 8 discordant strains. In addition, mobilized colistin resistance-1 (mcr-1) gene were detected in 5 colistin-resistant E. coli isolates with MIC of 8 mg/L. Conclusion: Both the RPNP and MicroScan systems demonstrated high categorical agreement with the rBMD method, underscoring their reliability as practical tools for routine colistin susceptibility testing in clinical microbiology laboratories where rBMD may not be feasible.