Comparative evaluation of colistin susceptibility testing by Disk Diffusion and Broth Microdilution methods: An experience from a tertiary care hospital.

Background: Recent times have seen uninhibited use of colistin due to emergence of carbapenem resistant gram-negative bacteria especially in India and the most common method still employed by most of the laboratories for in vitro testing of colistin is disk diffusion method.

Objective: The study aimed to compare two methods for colistin susceptibility testing Methods: This retrospective observational study was done on a total of 212 non-duplicate carbapenem resistant gram-negative isolates from patients attending our tertiary care hospital from April 2019 to June 2020. Colistin susceptibility testing for these isolates was done by disk diffusion method followed by broth microdilution method.

Results: We found out that disk diffusion method showed relatively low sensitivity [34.5%; 95%CI: 19.94-52.65] but high specificity [99.45%; 95% CI: 96.97 - 99.90] in detecting colistin resistant organisms in comparison to broth microdilution method.

Conclusion: Disk diffusion method is an unreliable method at detecting colistin resistance. Therefore, we should always rely on susceptibility testing by standard broth microdilution or newly introduced broth disk elution method before dispatching the report even in resource limited settings. The early and accurate reporting of susceptibility results can preserve the therapeutic value of the drug until we have newer treatment options available in the country.