Sarman Singh, Beenish Syed Rufai, Parveen Kumar and Jitender Singh
Division of Clinical Microbiology & Molecular Medicine, All India Institute of Medical Sciences, New Delhi, India
Tuberculosis is global health problem, specially after AIDS epidemic. India is having largest pool of drug resistant tuberculosis. The most important concern is its early diagnosis and drug resistance. The conventional methods of diagnosis and drug resistance detection are cumbersome, takes several weeks and lack reproducibility. Therefore in recent years several molecular methods such as conventional monoplex and multiplex polymerase chain reactions, real time PCR assays, Line probe assay and Xpert MTB/RIF have been made commercially available. The MTBDRplus line probe assay (LPA) and Xpert MTB/RIF have been endorsed by World Health Organization and both these tests can be used for detecting Tuberculosis as well as for drug resistant detection in Mycobacterium tuberculosis. However, there is no clarity regarding the superiority of one over the other. Therefore, for the first time from India, we carried out a prospective study, to evaluated the efficacy of Xpert MTB/RIF and LPA on culture confirmed samples. A total of 405 sputa of suspected drug resistant tuberculosis patients were included. Of these, 285 samples were smear positive and all these were subjected to LPA. Seventy-two (25.8%) samples showed multi-drug resistance, 62 (22.2%) showed rifampicin monoresistance, 29 (10.3%) isoniazid monoresistance and 116 (41.5%) were pan-susceptible. All 62 rifampicin monoresistant samples detected by LPA were tested by Xpert MTB/RIF using cartridge version G4. Of these, 38 (61.4%) showed concordance with LPA showing rifampicin resistance while 21 (33.8%) were found discordant susceptible to rifampicin by Xpert MTB/RIF using cartridge version G4. Of the 116 pan-susceptible samples, only 83 were available for Xpert MTB/RIF testing; of which 4 (5.1%) were found rifampicin resistant, 74 (94.8%) were susceptible. The 25 discrepant samples were further subjected to MGIT960 drug-susceptibility testing. The MGIT960 results showed 100% agreement with LPA results but only 64.4% agreement with Xpert MTB/RIF results. Sequencing analysis of discrepant samples showed 91.3% concordance with LPA but only 8.7% concordance with Xpert MTB/RIF assay. These isolates were characterized by spoligotyping and mycobacterial interspersed repetitive-unit–variable-number-tandem-repeat (MIRU-VNTR) analysis. Interestingly, most of these strains were CAS (CAS-1_Delhi [SIT 26] and 2 of SIT 846), followed by Orphan (SIT 27), and only 1 belonged to the MANU2 (SIT 1976) genotype. NO isolate was Beijing type.
This study shows that Xpert MTB/RIF may not be the first choice of molecular test to be used as point of care as primary screening test.