Evaluation of a direct colorimetric assay for rapid detection of rifampicin resistant Mycobacterium tuberculosis

Abstract

Abstract Background: Susceptibility test is a useful tool for selecting and modifying appropriate treatment for tuberculosis. Objective: To evaluate a direct colorimetric assay based on 3-(4,5-dimethylthiazol-2-yl)- 2,5-diphenyl tetrazolium bromide (MTT) for a rapid detection of rifampicin resistance. Methods: Sputum was inoculated directly into 7H9 broth supplemented with PANTA (antibiotic mixture) and oleic acid albumin dextrose catalase (OADC) to which MTT was added after 1-3 weeks. Bacterial growth is indicated by the formation of a colored formazan product, which can be measured with a spectrophotometer. The assay was read on the first, second or third week of incubation depending on when a measurable formazan was observed in the drug free control tube. The results were compared against the standard indirect modified proportion method of susceptibility testing on 7H10 media. Single sputum samples from 78 re-treatment cases were tested. Results: The MTT assay was found to be 90.5% sensitive and 100% specific. In more than two-thirds of the samples, results were already available by the end of week two. Conclusion: The direct colorimetric assay is a simple rapid test for rifampicin susceptibility testing. It significantly shortens the time required to obtain a drug susceptibility test result and could be useful to screen for MDR-TB. [Ethiop.J.Health Dev. 2005;19(1):51-54]