A comparison of the E-test and proportion methods for susceptibility testing of Mycobacterium tuberculosis

The aim of this study was to investigate the correlation between three antibiotic susceptibility methods, the proportion method on Löwenstein Jensen medium (LJ medium), the proportion method on Middlebrook 7H11 agar (7H11 agar), and the E-test for Mycobacterium tuberculosis. Fifty M. tuberculosis isolates were tested in vitro against isoniazid, rifampicin, streptomycin, and ethambutol according to the E-test, the proportion methods on 7H11 agar and LJ medium and then compared with a reference test which was the proportion method on 7H11 agar. The correlations between proportion method on 7H11 agar and proportion method on LJ medium for isoniazid, rifampicin, streptomycin, and ethambutol were 93.9%, 85.1%, 85.4% and 78.7% respectively. The correlations between the proportion method on 7H11 agar and the E-test were 83.1%, 78.8%, 84.7% and 80.5% respectively. There were no significant differences observed between the E-test and LJ medium compared to 7H11 agar. The average times to obtain susceptibility test results were 7 and 21 days for the E-test and agar proportion methods, respectively. The E-test may be suitable for replacing the proportion methods (7H11 agar and LJ medium) in routine practice due to its fast and easy application.