Drug Susceptibility Testing of M. tuberculosis to Isoniazid and Rifampicin- A Comparison between MTT Tube Method and LJ Proportion Method

Aim: To evaluate the methodology of MTT tube assay and compare it with standard proportion method for detection of drug susceptibility of M. tuberculosis to rifampicin (RIF) and isoniazid (INH).
Study Design: Prospective.
Place and Duration of Study: Sher-i-Kashmir Institute of Medical Sciences, Kashmir, India. One year study.
Methodology: MTT [3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide] assay was performed on 60 clinical isolates of M. tuberculosis. An inoculum of 107CFU/ml prepared in Middlebrook 7H9 with OADC (Oleic acid, albumin, dextrose and catalase) was chosen as standard. For each drug three tubes were used, one drug containing (INH 0.2 µg/ml or RIF 1 µg/ml), second inoculum control and third blank control. The method was performed after incubating the tubes at 37°C for 4 days for RIF and 7 days for INH. Results were read visually and by spectrophotometer at 570 nm. Relative optical density units of 0.2, was taken as cutoff. Results of drug susceptibility were compared with those obtained by Lowenstein Jensen proportion method.
Results: For RIF, sensitivity was 88.9% and 94.4%; specificity was 100% and 97.6% for visual MTT and MTT by RODU respectively. For INH similar sensitivity of 95.1% was seen while specificity was 97.0% and 95.0% by visual MTT and MTT by RODU respectively. There was almost perfect agreement between proportion and MTT method for both drugs. Turn-around time for MTT assay was 7 days.
Conclusion: The MTT tube assay can be used for rapid drug susceptibility testing of M. tuberculosis to RIF and INH.