Xpert MTB/RIF Ultra CT Value Provides a Rapid Measure of Sputum Bacillary Burden and Predicts Smear Status in Patients with Pulmonary Tuberculosis

Traditionally, smear microscopy has been used to estimate bacillary burden in order to assess infectiousness in tuberculosis (TB) patients. Since Xpert MTB assays might replace smear microscopy as the first-line diagnostic test for pulmonary tuberculosis, an alternative measure of bacillary load that correlates with smear positivity is needed. This study assessed the correlation between C (with and without normalization), smear status, culture time-to-positivity (TTP), and clinical factors in patients with Xpert ultra positive sputum during a four-year period. A cut-off C value for smear positivity was also estimated. 204 samples were included. Strong correlation between both Xpert Ultra C values (raw and normalized) and smear status was obtained (r = 0.78 and - 0.79, respectively). The association between Raw-C and TTP was weaker than normalized-C (N-C) and TTP (r = 0.50 and r = - 0.70, respectively). A Raw-C cut-off value of 21.4 was identified with 85.7% (95% CI 65.4-95) sensitivity and 92.9% (95% CI 84.3-96.9) specificity. A N-C cut-off value of 5.2 yielded a sensitivity of 94.3% (95% CI 86.2-97.8) and specificity of 85.7% (95% CI 65.4-95). Our study demonstrates that Xpert Ultra C value correlates well with other measures of bacillary load such as smear status or TTP. The correlation with TTP is stronger when the C value is normalized using the internal control. The proposed N-C cut-off value of 5.2 shows a better sensitivity than the Raw-CT when predicting smear positive status.