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% CO,) 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.