Challenges & issues of colistin susceptibility testing in diagnostic microbiology laboratories

Sir,

I read with interest the article by Sinha et al1 on colistin-resistant bacteria. The authors have highlighted the pertinent issue of drug-resistant Gram-negative bacilli in intensive care unit (ICU) settings of a tertiary care centre in India, which is now a global phenomenon. I would like to highlight some of the points in the above-mentioned article:

1. A joint initiative of the European Centre for Disease Prevention and Control and the US Centers for Disease Control and Prevention had published expert consensus definition of multidrug resistant (MDR), extensively drug resistant (XDR) and pandrug-resistant (PDR) bacterial agents2. According to that definition, an isolate can be levied as PDR if it is non-susceptible to all agents in all the antimicrobial categories in current clinical use. The authors have not mentioned whether all the classes of drugs were tested.

2. The authors have determined colistin minimum inhibitory concentration (MIC) by E-test using Colistin Ezy MIC™ Strip (HiMedia Laboratories Pvt. Ltd., Mumbai). It is a known fact that colistin and polymyxin B E-test is associated with high major errors3. The implementation of the Clinical and Laboratory Standards Institute (CLSI)-approved broth microdilution method in routine microbiology laboratories remains a challenge till date.

In conclusion, there is no second thought that the judicious use of colistin is the need of the hour, but every microbiology laboratory which is catering to ICU patients receiving polymyxin B/colistin should adopt the standardized CLSI/EUCAST (European Committee on Antimicrobial Susceptibility Testing) method of colistin susceptibility testing at least in MDR/XDR isolates3.