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BackgroundPatients at elevated risk of drug-resistant tuberculosis (TB) are prioritized for Xpert(®) MTB/RIF testing; however, the clinical usefulness of the test in this population is understudied.DesignFrom November 2011 to June 2014, consecutive out-patients with a history of previous TB in high-density suburbs of Harare, Zimbabwe, were tested using Xpert, solid and liquid culture, and the microscopic observation drug susceptibility assay. Diagnostic accuracy for rifampin (RMP) resistance and time to initiation of second-line regimens were ascertained. The rpoB gene was sequenced in cases with culture-confirmed RMP resistance and genotypic susceptibility.ResultsAmong 352 retreatment patients, 71 (20%) were RMP-resistant, 98 (28%) RMP-susceptible, 64 (18%) culture-negative/Xpert-positive, and 119 (34%) culture-negative/Xpert-negative. Xpert had a sensitivity of 86% (95%CI 75-93) and a specificity of 98% (95%CI 92-100) for RMP-resistant TB. The positive predictive value of Xpert-determined RMP resistance for multidrug-resistant TB (MDR-TB) was 82% (95%CI 70-91). Of 71 (83%) participants, 59 initiated treatment with second-line drugs, with a median time to treatment initiation of 18 days (IQR 10-44).ConclusionThe diagnostic accuracy of Xpert for RMP resistance is high, although the predictive value for MDR-TB was lower than anticipated. Xpert allows for faster initiation of second-line treatment than culture-based drug susceptibility testing under programmatic conditions.

Abstract Rifampicin (RMP) resistant strains are still persisting as a threat to the global TB control program. Therefore, understanding the RMP resistant mechanism is need of the hour. The current study has investigated the role of each RMP binding site (RBS) residues, deploying computational alanine scanning mutagenesis (CASM) to unravel the critical and non-critical binding site residues. In addition, conformational shifts in RBS cavity of different RNAP β-subunit (RpoB) systems have also been analyzed. Our initial findings showed that in addition to reported mutational sites, Q510, Q513, R529, P564 and P566 were also critical binding site residues, which upon mutation destabilize the RMP binding. Study also indicated that R3, R4 and R5 regions of RpoB were very significant for the functioning of RMP. The concerted interactions of these regions with RMP hold it into the centre position of RBS and restrict the entrance of elongating RNA transcript. Whereas, after mutation, due to repositioning of RMP and changes in its interactions, the overall cavity becomes significantly hollow which may confer space for phosphodiester bond formation during transcription elongation. Our findings provide valuable details to forestall RMP resistance and may help in the development of new leads against the RMP resistant strains. [ABSTRACT FROM AUTHOR]

Setting: In Zimbabwe, there are concerns about the management of tuberculosis (TB) patients with rifampicin (RMP) resistance diagnosed using Xpert(®) MTB/RIF., Objective: To assess linkages between diagnosis and treatment for these patients in Harare and Manicaland provinces in 2014., Design: A retrospective cohort study., Results: Of 20 329 Xpert assays conducted, 90% were successful, 11% detected Mycobacterium tuberculosis and 4.5% showed RMP resistance. Of 77 patients with RMP-resistant TB diagnosed by Xpert, 70% had samples sent to the reference laboratory for culture and drug susceptibility testing (CDST); 53% of the samples arrived. In 21% the samples showed M. tuberculosis growth, and in 17% the DST results were recorded, all of which confirmed RMP resistance. Of the 77 patients, 34 (44%) never started treatment for multidrug-resistant (MDR) TB, with documented reasons being death, loss to follow-up and incorrect treatment. Of the 43 patients who started MDR-TB treatment, 12 (71%) in Harare and 17 (65%) in Manicaland started within 2 weeks of diagnosis., Conclusion: Xpert has been rolled out successfully in two Zimbabwe provinces. However, the process of confirming CDST for Xpert-diagnosed RMP-resistant TB works poorly, and many patients are either delayed or never initiate MDR-TB treatment. These shortfalls must be addressed at the programmatic level.