Evaluation of Colistin Broth Disk Elution and Colistin Agar Test: A study from Tertiary Care Hospital, South India

Enterobacterales particularly which are resistant to carbapenem group of antibiotics (CRE) are precariously being reported worldwide. Last option for treating the infections caused by CRE are polymyxin E (Colistin) and polymyxin B. Resistance to polymyxins is on higher side because of its increased usage both clinically and non-clinically. In vitro evaluation tests for susceptibility of colistin is associated with lot of complexities due to its innate cationic properties. Hence it is essential for all diagnostic laboratory to standardize colistin testing method, so the present study was undertaken to evaluate the results of colistin broth disk elution (CBDE) and colistin agar test (CAT) in comparison with the reference broth microdilution (rBMD). About 100 CRE clinical isolates were tested, results of CBDE & CAT was compared with rBMD. Categorical agreement (CA) of CBDE was 98% with 2% of very major error (VME), CA of CAT was 99% with 1% of VME in comparison with rBMD. Because of increasing colistin resistance it is crucial to report colistin MIC with a validated method, so we would like to recommend CAT test for routine MIC reporting of colistin since it is feasible test.