Evaluation of polymyxin B epsilometer test susceptibility to multidrug-resistant Gram-negative clinical isolates at a tertiary care center

Background: Multidrug-resistant (MDR) Gram-negative bacilli that release carbapenemase enzymes ensure that they are not affected by many beta-lactam antibiotics and carbapenems. The methodology of testing susceptibility to polymyxin B can pose a challenge in a clinical scenario. Aims: To assess polymyxin B, E -test susceptibility against Gram-negative bacteria that are MDR and compare the results of it with broth micro dilution (BMD). Settings and Design: This prospective observational study was done at a tertiary care center laboratory for 1 year. Materials and Methods: Polymyxin B susceptibility against 74 Gram-negative clinical isolates were assessed using the E -test and BMD as per the Clinical Laboratory Standards Institute recommendations. Statistical Analysis: Descriptive statistics of frequency, percentage, median, and intra-quartile range were done. Kolmogorov–Smirnov test was performed to assess the normality of the numerical variables. Results: The predominant organisms isolated were Klebsiella pneumoniae (55.4%) followed by Pseudomonas aeruginosa (21.6%), Acinetobacter baumanni complex (16.2%), Escherichia coli (4.1%), and Enterobacter cloacae (2.7%). The polymyxin B susceptibility ranges were much higher by E -test method when cross checked with BMD (97% as against 77%; with P < 0.0001). E -test however correlated poorly with BMD. Conclusion: BMD testing proved to be a more reliable method in situations where the clinical response to infection was inadequate or compromised.