Evaluation of E test for susceptibility testing of Mycobacterium tuberculosis to primary anti tubercular drugs.

BACKGROUND & OBJECTIVES: Antimicrobial susceptibility tests for tuberculosis take weeks and delayed therapy can lead to an increase in disease incidence. The E test is a new concept for minimum inhibitory concentrations (MIC) determinations for antimicrobial agents that is based on a predefined antibiotic gradient on a plastic strip calibrated with a continuous logarithmic MIC scale covering 15 two-fold dilutions. The present study was undertaken to evaluate E test strips for susceptibility testing of Mycobacterium tuberculosis. METHODS: Twenty five clinical isolates of M. tuberculosis were tested for the four first line antitubercular drugs by E test and were compared with standard proportion method. The inoculum turbidity was adjusted to McFarland 3.0 standard and agar plates (Middle brook 7H11 agar) were inoculated and preincubated (37 degrees C in 7-10% CO2) for 24 h after which time, the E test strips were placed on the agar surface which were incubated under same conditions. The MIC was interpreted as the point at which the ellipse intersected the 'E test' strip as described in E test technical guide. RESULTS: Of the 25 strains, susceptibility as determined by both methods for isoniazid (INH), rifampin, ethambutol and streptomycin was found in 22 (88%), 20 (80%), 24 (96%) and 18 (72%) strains respectively. Agreement between E test and proportion method was 96 per cent for INH, 92 per cent for rifampin and 100 per cent for ethambutol and streptomycin each. However, sensitivity could be predicted after 7-10 days by E test and exact MIC could also be determined. INTERPRETATION & CONCLUSIONS: E test method was found to be rapid, accurate, reliable and easy to perform. It can be employed for routine susceptibility testing for antitubercular drugs.