Xpert MTB/RIF Cycle Threshold as a Marker of Tuberculosis (TB) Disease Severity: Implications for TB Treatment Stratification

BACKGROUND: Recent trials have demonstrated that shortened 4-month treatment durations are effective for the majority of people with tuberculosis (TB). However, there is a population of patients with TB 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 analyzed data from the RIFASHORT TB treatment-shortening noninferiority trial to define a TB phenotype classification. The RIFASHORT trial primary outcome was reanalyzed using the protocol-defined noninferiority criterion of 8 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 nonrelapse. 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 posttreatment 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 1200-mg rifampicin-containing regimen met the protocol-defined noninferiority criterion in both modified intention-to-treat (adjusted risk difference: -1.3%; 95% CI, -6.7% to 4.0%) and per protocol analyses (1.7%; 95% CI, -3.8% to 7.1%). CONCLUSIONS: The TB phenotype classification derived here successfully identified three-quarters of RIFASHORT trial participants for whom a 4-month 1200-mg rifampicin regimen was noninferior to the 6-month standard of care. A definitive phase III randomized trial of disease-stratified rifampicin-based TB treatment is justified.