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 µg ml−1 of polymyxin B) and agar dilution (antibiotic serially diluted 0·25–64 µg ml−1). CRE of 42·3% were resistant to polymyxin B (MICs range: 0·25–>64 µg ml−1) and 16·8% presented borderline MICs. Sensitivity, specificity, PPV and NPV were 86·2, 98·7, 98 and 90·7% for screening test and 86·2, 97·5, 96·1 and 90·6% for agar dilution. ME was 0·73 and 1·5% for screening and agar dilution respectively; VME was 5·8% 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. Significance and Impact of the Study: Determining polymyxins susceptibility in clinical laboratories is challenging. Disk diffusion and antimicrobial gradient assays are not accurate enough for these antibiotics. Broth microdilution (BMD) is time consuming and laborious. We evaluated polymyxin B agar‐based methods as an alternative to BMD. Agar dilution and agar screening tests presented good performances. As agar screening test is less laborious and cheaper than agar dilution, it may be an option in situations where the absence of polymyxin B MIC is clinically acceptable. As far as we know, this is the first study that evaluates agar screening with polymyxin B instead of colistin.