Discordance in Xpert® MTB/RIF assay results among low bacterial load clinical specimens in Bangladesh.

BACKGROUND: The Xpert® MTB/RIF assay detects Mycobacterium tuberculosis (MTB) and rifampicin (RIF) resistance. RIF-resistant (RIF-R) MTB cases detected using Xpert on sputum specimens at three private-sector TB screening centres in Dhaka, Bangladesh, were subjected to consecutive confirmatory Xpert testing, the results of which were MTB-positive/RIF-susceptible, MTB-positive/RIF-indeterminate or MTB-negative. OBJECTIVE: To assess the possible causes of discordant MTB and RIF-R results. METHODS: Discordant confirmatory Xpert test results were subjected to further investigations using the GenoType® MTBDRplus assay, culture and rpoB gene sequencing. RESULTS: The confirmatory Xpert test was performed on a remnant or a second specimen collected from individuals with an initial RIF-R result (n = 69); 22 (32%) results were discordant, 20 of which had an 'MTB detected-very low' result. Further investigations were mostly concordant with the confirmatory Xpert test. Average variability in paired cycle threshold (Ct) values were higher in 'MTB detected-very low' results vs. specimens with low, medium or high detected MTB results (P < 0.05); discordant results were mostly observed in specimens with 'MTB detected-very low' (20/22). CONCLUSIONS: Repeating the Xpert test and comparing with other available tests should be considered in case of 'MTB detected-very low, RIF resistance detected' results on Xpert.

Molecular epidemiology of tuberculosis in rural Matlab, Bangladesh.

OBJECTIVE: To characterise and classify clinical isolates collected from tuberculosis (TB) patients in rural Bangladesh and to investigate the mode of transmission. DESIGN: An epidemiological study using a combination of conventional and molecular methods was performed in a rural population of Bangladesh. A total of 168 clinical isolates were collected from TB patients. Deletion analysis, used for rapid differentiation of members of the Mycobacterium tuberculosis complex, spoligotyping and variable number tandem repeats of mycobacterial interspersed repetitive units (VNTR-MIRU) typing were used. RESULTS: Deletion analysis identified all isolates as M. tuberculosis and further divided them into 109 strains (65%) carrying the M. tuberculosis deletion region 1 (TbD1-intact or 'ancestral' strains) and 59 strains (35%) lacking this region (TbD1 or 'modern' strains). MIRU analyses showed that 149 strains (89%) had unique patterns, whereas 19 strains (11%) clustered into eight groups. The largest cluster comprised five TbD1 strains of the Beijing type. The rate of recent transmission was estimated to be 6.5%. CONCLUSIONS: Our results suggest that TB in rural Bangladesh is caused primarily by reactivation of latent infections involving TbD1 intact strains, overlaid with the recent emergence of Beijing strain clusters that include multidrug-resistant isolates.