A new rapid colourimetric method for testing Mycobacterium tuberculosis susceptibility to isoniazid and rifampicin: a crystal violet decolourisation assay

The aim of this study was to investigate the performance of a new and accurate methodfor the detection of isoniazid (INH) and rifampicin (RIF) resistance amongMycobacterium tuberculosis isolates using a crystal violetdecolourisation assay (CVDA). Fifty-five M. tuberculosis isolatesobtained from culture stocks stored at -80ºC were tested. After bacterialinoculation, the samples were incubated at 37ºC for seven days and 100 µL of CV (25mg/L stock solution) was then added to the control and sample tubes. The tubes wereincubated for an additional 24-48 h. CV (blue/purple) was decolourised in thepresence of bacterial growth; thus, if CV lost its colour in a sample containing adrug, the tested isolate was reported as resistant. The sensitivity, specificity,positive predictive value, negative predictive value and agreement for INH were92.5%, 96.4%, 96.1%, 93.1% and 94.5%, respectively, and 88.8%, 100%, 100%, 94.8% and96.3%, respectively, for RIF. The results were obtained within eight-nine days. Thisstudy shows that CVDA is an effective method to detect M.tuberculosis resistance to INH and RIF in developing countries. Thismethod is rapid, simple and inexpensive. Nonetheless, further studies are necessarybefore routine laboratory implementation.