Comparative Evaluation of Rapid Polymyxin Nordmann-Poirel Test and VITEK-2 for Colistin Resistance in Escherichia coli and Klebsiella pneumoniae

Introduction: Escherichia coli (E.coli) and Klebsiella pneumoniae (K.pneumoniae) are the most common organisms isolated from clinical specimens. Increasing Multi-Drug Resistance (MDR) in these Gram-negative organisms has caused considerable therapeutic challenges in clinical practice. This has recently revived interest in colistin as a remedial option. The increasing use of colistin entails the provision of rapid and reliable methods for colistin susceptibility testing. Aim: To evaluate the performance of rapid polymyxin Nordmann-Poirel (NP) test and VITEK-2 for detection of colistin resistance. Materials and Methods: Two methods of colistin susceptibility testing, VITEK-2 and Rapid polymyxin NP test were compared on 310 isolates of E.coli and K.pneumoniae obtained from various clinical samples such as blood, urine, sputum, pus and endotracheal aspirates. Kappa analysis was done to evaluate the percentage agreement between Vitek-2 and Rapid Polymyxin NP tests. Results: Out of 310 isolates, 232 were E.coli and 78 were K. pneumoniae. The results of both the methods was consistent in (237/310) 76.45% cases and non-consistent in (73/310) 23.55% cases. Kappa analysis revealed that the strength of agreement between the two test was considered moderate (kappa=0.412, confidence interval: from 0.312 to 0.513). Conclusion: Rapid polymyxin NP test when compared to VITEK-2 was simple, rapid and cost-effective. Thus, it can be used in laboratories for screening of polymyxin resistance in carbapenemase-producing Gram-negative bacteria as recommended by CLSI (Clinical Laboratory Standard Institute).