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

Abstract

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.

Dilemmas with ethionamide susceptibility testing of Mycobacterium tuberculosis: A microbiologist & physician′s nightmare

“…Testing for drug susceptibility to ethionamide, a crucial part of second line anti-tuberculosis therapy, is problematic as the difference characterizing drug resistance or susceptibility is small. Moreover, the minimum inhibitory concentration (MIC) is very near to the therapeutic index as commented by Lakshmi et al, in their article in this issue 4 . In addition, the drug is thermolabile, which makes drug susceptibility testing more difficult 5 .…”

“…For a drug like ethionamide, where drug susceptibility pattern is difficult to determine and the therapy is empirical, development of resistance may be a serious cause of concern. Lakshmi et al 4 in this issue have thus taken up a crucial investigation in the wake of emergence of MDR-TB and extensively drug resistant tuberculosis (XDR-TB).…”

“…The World Health Organization (WHO) has defined critical concentrations for various second line drugs taking into account the in vitro criteria for resistance using representative clinical samples and recommended a concentration of 40 µg/ml for proportion method of drug susceptibility testing of ethionamide 6 7 . Lakshmi et al 4 also used the critical concentration of 40 µg/ml to perform the drug susceptibility assay by proportion method. However, the thermolabile nature of ethionamide may play a role in reducing the potency of the drug during inspissation of the drug containing medium.…”

“…Deterioration of the drug during prolonged incubation period required for M. tuberculosis can also be crucial. Technical errors, as mentioned by Lakshmi et al 4 like inoculum preparation, incubation and interpretation can add to erroneous results and should be minimized.…”

“…Mpagama et al 3 used 5 µg/ml of ethionamide as the critical concentration to segregate borderline susceptible from resistant through MIC on MycoTB sensititer plates (Trek Diagnostics, Cleveland, Ohio, USA). Lakshmi et al 4 recommended a much higher concentration of 80 µg/ml for MIC on Lowenstein-Jensen (L-J) medium. The sensitivity of the MIC improved using 80 µg/ml as the cut-off.…”