Xpert MTB/RIF® cycle threshold as a marker of TB disease severity; Implications for TB treatment stratification

Abstract Introduction Recent trials have demonstrated that shortened four-month treatment durations are effective for the majority of people with tuberculosis (TB). However, there is a population of TB patients who require longer treatment durations. Prospectively identifying those who require shorter versus longer treatment durations would support evaluation and implementation of optimized regimens. Methods We analysed data from the RIFASHORT TB treatment-shortening non-inferiority trial to define a TB phenotype classification. The RIFASHORT trial primary outcome was reanalysed using the protocol-defined non-inferiority criterion of eight percentage points, stratifying by those classified as having limited or extensive disease. Results Xpert MTB/RIF® semiquantitative bacterial burden in combination with TB disease involvement grading on chest X-ray achieved the strongest differentiation between relapse and non-relapse. The extensive disease TB phenotype (high semiquantitative bacterial burden and extensive TB disease on X-ray) accounted for one quarter of the RIFASHORT trial population and more than half of all post-treatment TB relapses (13/23). For the limited TB disease phenotype (a semiquantitative bacterial burden other than high or no extensive TB disease on X-ray), the experimental 4-month 1200mg rifampicin-containing regimen met the protocol-defined non-inferiority criterion in both modified-intention-to-treat (adjusted risk difference: -1.3% (95% confidence interval: -6.7% to 4.0%)) and per protocol analyses (1.7% (95% CI: -3.8% to 7.1%)). Conclusion The TB phenotype classification derived here successfully identified three-quarters of RIFASHORT trial participants for whom a four-month 1200mg rifampicin regimen was non-inferior to the six-month standard of care. A definitive phase III randomised trial of disease-stratified rifampicin-based TB treatment is justified.