Performance Of Polymyxin B Agar-Based Tests Among Carbapenem-Resistant Enterobacterales

Therapeutic options for infections caused by Carbapenem-resistant Enterobacterales (CRE) are restricted and include polymyxins-centred schemes. Evaluation of in vitro susceptibility is difficult and time consuming. Agar-based methodologies are an alternative to broth microdilution (BMD) and we aimed to evaluate the accuracy of those methods among Enterobacterales. A total of 137 non-duplicated CRE were subjected to polymyxin B BMD, agar screening test (Mueller Hinton plates containing 3 mu g ml(-1) of polymyxin B) and agar dilution (antibiotic serially diluted 0 center dot 25-64 mu g ml(-1)). CRE of 42 center dot 3% were resistant to polymyxin B (MICs range: 0 center dot 25->64 mu g ml(-1)) and 16 center dot 8% presented borderline MICs. Sensitivity, specificity, PPV and NPV were 86 center dot 2, 98 center dot 7, 98 and 90 center dot 7% for screening test and 86 center dot 2, 97 center dot 5, 96 center dot 1 and 90 center dot 6% for agar dilution. ME was 0 center dot 73 and 1 center dot 5% for screening and agar dilution respectively; VME was 5 center dot 8% for both techniques. In general, agar-based methods had a good performance. As far as we know, this is the first study to propose an agar screening test using polymyxin B instead of colistin.