Revisiting the susceptibility testing of Mycobacterium tuberculosis to ethionamide in solid culture medium

• Main Authors: Rajagopalan Lakshmi, Ranjani Ramachandran, D Ravi Kumar, A Syam Sundar, G Radhika, Fathima Rahman, N Selvakumar, Vanaja Kumar
• Format: Article
• Language: English
• Published: Wolters Kluwer Medknow Publications 2015-01-01
• Series: Indian Journal of Medical Research
• Subjects: Drug resistance - ethionamide - MIC method - Mycobacterium tuberculosis - proportion sensitivity testing
• Online Access: http://www.ijmr.org.in/article.asp?issn=0971-5916;year=2015;volume=142;issue=5;spage=538;epage=542;aulast=Lakshmi

Background & objectives: Increase in the isolation of drug resistant phenotypes of Mycobacterium tuberculosis necessitates accuracy in the testing methodology. Critical concentration defining resistance for ethionamide (ETO), needs re-evaluation in accordance with the current scenario. Thus, re-evaluation of conventional minimum inhibitory concentration (MIC) and proportion sensitivity testing (PST) methods for ETO was done to identify the ideal breakpoint concentration defining resistance. Methods: Isolates of M. tuberculosis (n=235) from new and treated patients were subjected to conventional MIC and PST methods for ETO following standard operating procedures. Results: With breakpoint concentration set at 114 and 156 µg/ml, an increase in specificity was observed whereas sensitivity was high with 80 µg/ml as breakpoint concentration. Errors due to false resistant and susceptible isolates were least at 80 µg/ml concentration. Interpretation & conclusions: Performance parameters at 80 µg/ml breakpoint concentration indicated significant association between PST and MIC methods.