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.