Polymyxin B broth disk elution as a screening test to determine polymyxin B susceptibility in enterobacterales

Abstract

Detection of polymyxins susceptibility by clinical laboratories is a nightmare mainly because of physicochemical properties of the drug. The aim of this study was to evaluate accuracy of Polymyxin B Broth Disk Elution (PBDE), as screening test, compared to Broth Microdilution (BMD). We evaluated 196 Enterobacterales resistant to carbapenems. BMD was performed as standardized by EUCAST and CLSI. PBDE was performed in 15-mL cation-adjusted Mueller-Hinton broth where one polymyxin B disk (300 IU) was eluted (2 μg/mL). Results were interpreted based on visual turbidity. Categorical Agreement (CA) was 99.5%, sensitivity and specificity were 98.9% and100%, respectively, after 16-20 hours of incubation. Sensitivity of PBDE with shorter incubation time were 63.6%, 69.7% and 78.8% after 6h, 7h and 8h, respectively. Those sensitivities were mainly compromised by isolates presenting borderline MICs. PBDE is a cheap and easy to perform methodology as reliable as BMD to evaluate the susceptibility to polymyxin B among Enterobacterales.