Evaluation of line-probe assay for molecular analysis and drug susceptibility of extra-pulmonary tuberculosis

Setting Most epidemiological studies on Mycobacterium tuberculosis focus on pulmonary tuberculosis (TB), whereas extra-pulmonary TB (EPTB) remains poorly explored. Objective To study the rate of resistant EPTB cases among individuals with suspected EPTB using a commercial line-probe assay (LPA), polymerase chain reaction (PCR) and reverse hybridisation test. We also examined the molecular profile of the EPTB isolates obtained at the Himalayan Institute of Medical Sciences, Dehradun, India. Design EPTB samples were collected from 249 patients with clinical and radiological suspicion of EPTB and subjected to automated liquid culture, PCR and GenoType MDRTBplus according to the manufacturers' instructions. Results A diagnostic yield of 15% was observed among individuals with suspected EPTB using MGIT™ (Mycobacterium Growth Indicator Tubes), which increased to 38% on LPA and PCR. LPA results had 100% concordance with MGIT, with all culture-positive samples also being positive on LPA. However, 70.2% of LPA-positive samples did not grow Mycobacterium tuberculosis in liquid culture. Two (2.1%) of the culture-negative EPTB PCR-positive samples were multidrug-resistant, 20 (21.2%) were rifampicin-monoresistant and 12 (12.7%) isoniazid-monoresistant on LPA. Conclusion Given the paucibacillary nature of EPTB, we demonstrated that PCR and LPA can have a vital role in establishing TB diagnosis in extra-pulmonary tissues.