EVALUATION OF THE PERFORMANCE OF THE NITRATE REDUCTASE AND RESAZURIN TITRE ASSAY FOR THE DETECTION OF MYCOBACTERIUM TUBERCULOSIS COMPLEX FROM SPUTUM IN A HIGH TB AND HIV SETTING

Tuberculosis (TB) remains a leading cause of death in developing countries and its burden has been exacerbated by the concurrent HIV epidemic. Despite the advances in medicine, TB diagnosis still remains a challenge, especially in developing countries where diagnosis relies mostly on the detection of Mycobacterium tuberculosis complex (MTBC) by smear microscopy and/or culture. Smear microscopy is rapid, simple and not expensive but it lacks sensitivity. Culture on solid medium, which is performed in some well equipped laboratories, is more sensitive than microscopy but takes up to 8 weeks to obtain the result.

Colorimetric methods have been used for the rapid detection of drug sensitivity in M. tuberculosis either from isolates or directly from sputum. These methods rely on the detection of live bacteria through either enzymatic activity (nitrate reduction) or their ability to reduce an oxidation-reduction indicator, either in solid or liquid medium. They are fast, simple, and offer a good potential that should be evaluated for the diagnosis of TB.

The objective of this study is to evaluate the performance of colorimetric assays for the detection of M. tuberculosis complex from sputum samples from adult pulmonary TB suspects in a high TB and HIV prevalence setting, using Löwenstein Jensen (LJ) and Mycobacterium growth indicator tube (MGIT) culture as gold standard. The colorimetric methods evaluated here will be the solid medium-based nitrate reductase assay as described (NRA) or with an additional step using para Nitrobenzoic (PNB) acid for differentiation of MTBC and NTM (NRA-p), and the modified resazurin microplate assay, also using PNB for differentiation of MTBC and NTM (REMA-p).

. If any of the colorimetric assays is found to be accurate, significantly faster than conventional culture methods, and easy to perform, then it could be implemented in a tuberculosis culture laboratory. By reducing the time to detection compared to conventional culture and the costs compared to recent commercial methods, these assays offer a good alternative to conventional methods and might help to improve TB diagnosis in developing countries.