Polymyxin B Etest Fails to Distinguish Carbapenem-resistant Enterobacteriaceae with Elevated Polymyxin B MICs

BACKGROUND: Polymyxins are being revitalized to combat carbapenem-resistant Enterobacteriaceae (CRE). However, evaluating the activity of these agents by traditional broth dilution methods is not practical for busy clinical laboratories. We compared polymyxin B (PMB) activity utilizing two quantitative susceptibility testing methods, Etest® and broth microdilution (BMD), against CRE isolates from patients at an academic medical center. METHODS: PMB activity against 70 recent CRE clinical isolates was determined by BMD and Etest(®) according to CLSI guidelines. P. aeruginosa ATCC(®) 27853 was used as a quality control strain. The CLSI PMB susceptibility breakpoint of non-fermenting gram-negative bacteria (<2 mg/L) was used. Essential agreement between methods was defined as an MIC measured within 1 log(2) dilution. Categorical agreement was defined between methods as classification of isolates in the same susceptibility category (susceptible or resistant). Major and very major error rates were calculated, and McNemar’s test was used for determining a difference between methods. RESULTS: CRE isolates were primarily Enterobacter spp. (43%), followed by K. pneumoniae (41%) and E. coli (9%). Essential agreement between testing methods was low (9%), but categorical agreement was 81% (P = 0.0002). Although false non-susceptibility was never observed by Etest(®) (BMD as reference), the rate of very major errors by Etest(®) was high (19%). Etest(®) miscalled 87% of PMB-resistant CRE. CONCLUSION: Etest(®) reporting of false susceptibility may result in inappropriate antibiotic utilization and treatment failure clinically. We do not recommend using Etest® for PMB susceptibility testing for routine patient care. DISCLOSURES: All authors: No reported disclosures.