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.
© 2023. The Author(s).