Rapid Diagnosis of Extensively Drug-Resistant Tuberculosis by Use of a Reverse Line Blot Hybridization Assay

Drug resistance in tuberculosis (TB) is a matter of grave concern for TB control programs, as there is currently no cure for some extensively drug-resistant (XDR) strains. There is concern that this resistance could transmit, stressing the need for additional control measures, rapid diagnostic methods, and newer drugs for treatment. We developed an in-house assay that can rapidly detect resistance to drugs involved in the definition of XDR-TB directly from smear-positive specimens. Two hundred fifteen phenotypically XDR-TB isolates and 50 pansusceptible isolates were analyzed using a reverse line blot hybridization (RLBH) assay. The assay was also successfully applied to 73 smear-positive clinical specimens. The RLBH assay exhibited good sensitivity for the detection of resistance to isoniazid (99%), rifampin (99%), fluoroquinolones (95.3%), and second-line aminoglycosides (94.8%). The results from application of this assay on direct smear-positive clinical specimens revealed 93% concordance with the phenotypic drug susceptibility test (DST) results for the above-mentioned drugs. The time to accurate DST results was significantly reduced from weeks to 3 days. This molecular assay is a highly accurate tool for screening for XDR-TB, which achieves a substantial reduction in diagnostic delays.