Rapid antibiotic susceptibility testing of Mycobacterium tuberculosis: its utility in resource poor settings

Purpose: To compare the rapid colorimetric nitrate reductase based antibiotic susceptibility (CONRAS) test performed on Mycobacterium tuberculosis isolates with the conventional method i.e., the proportion method. Methods: One hundred clinical isolates of M. tuberculosis were tested for susceptibility to isoniazid (INH) and rifampicin (RIF) by the conventional proportion method and CONRAS in Middlebrook 7H9 liquid medium enriched with growth supplements (MB7H9S). Results: The performance of the CONRAS test was evaluated using proportion method as the gold standard. The sensitivity (ability to detect true drug resistance) and specificity (ability to detect true drug susceptibility) of the CONRAS test to INH was 93.75 and 98.52% and for RIF it was 96.10 and 100% respectively. The mean time for reporting was 6.3 days and the test showed excellent reproducibility. The kappa (κ) value for INH was 0.92 and for RIF was 0.99, indicating excellent agreement between the two methods. Conclusions: CONRAS test is a rapid and reliable method of drug susceptibility for M. tuberculosis.