Comparison of Antimicrobial Susceptibility Testing Methods for Colistin against Carbapenem Resistant Enterobacteriaceae in a Tertiary Care Hospital of Southern India

Abstract

Introduction: The emergence of resistance to colistin urges the need for standardised invitro susceptibility testing methods for colistin, both for patient care and also for epidemiological purpose. Many challenges have been faced by clinical laboratories in performing and interpreting Antimicrobial Susceptibility Testing (AST) methods for colistin. To overcome the challenges, a rapid and reliable method is required to carry out AST for colistin. Aim: To determine the degree of agreement and types of errors associated with various colistin antimicrobial susceptibility methods such as commercial Broth Microdilution (BMD) method (MIKROLATEST- Erba Mannheim colistin MIC kit) and in-house Rapid polymyxin NP test (named after P. Nordmann and L. Poirel) with the reference in-house BMD method. Materials and Methods: The study was carried out in a tertiary care hospital in southern India from July 2018 to July 2019. The isolates from Enterobacteriaceae family showing resistance to carbapenem were selected for the study. A total of 294 clinical isolates were collected and subjected to two test methods (Commercial BMD method and Rapid polymyxin NP test) and finally the comparative analytical study was done by comparing the results of the test methods with reference in-house BMD method. The statistical analysis was carried out with the help of Stata version 14 using kappa statistics. Results: Susceptibility results of 294 isolates included Escherichia coli (n=67), K.pneumoniae (n=195), Enterobacter spp. (n=23), Citrobacter spp. (n=9) were evaluated for three methods according to European Committee on Antimicrobial Susceptibility Testing (EUCAST) published clinical breakpoints. Overall, categorical agreement of commercial BMD with reference in-house method was 91.1% with very major error 2.7% and major error 6.1% and essential agreement was 83.3%. With the Rapid polymyxin NP test, categorical agreement, sensitivity and specificity were 92.5%, 93.1% and 92.3% respectively. Conclusion: This study helps in introducing less laborious, faster and reliable susceptibility method for colistin. This will guide the clinicians in appropriate and judicious use of colistin so that mortality and morbidity will be reduced drastically.

Keywords

• broth microdilution
• categorical agreement
• essential agreement
• minimal inhibitory concentration