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No abstract

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 CT (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 CT value for smear positivity was also estimated. 204 samples were included. Strong correlation between both Xpert Ultra CT values (raw and normalized) and smear status was obtained (r = 0.78 and − 0.79, respectively). The association between Raw-CT and TTP was weaker than normalized-CT (N-CT) and TTP (r = 0.50 and r = − 0.70, respectively). A Raw-CT 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-CT 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 CT value correlates well with other measures of bacillary load such as smear status or TTP. The correlation with TTP is stronger when the CT value is normalized using the internal control. The proposed N-CT cut-off value of 5.2 shows a better sensitivity than the Raw-CT when predicting smear positive status.

Traditionally, smear microscopy has been used as to estimate bacillary burden in order to assess infectiousness in tuberculosis patients. Xpert Ultra has the potential to replace smear. However, there are scarce data available to support its use as a quantitative marker. This study assessed the correlation between CT (with and without normalization), smear, culture time-to-positivity (TTP), and clinical factors in patients with Xpert ultra positive sputum during a four-year period. A cut off CT value for smear positivity was also determined. Two hundred and four samples were included. Strong correlation between both Xpert Ultra CT values (raw and normalized) and smear grade was obtained (r = 0.78 and − 0.79, respectively). The association between CT and TTP was weaker than smear (r = 0.50) but strong correlation (-0.70) between TTP and normalized- CT (N- CT) was observed. A cut off N- CT value of 5.2 was found to best predict smear positivity. Only cavitation and cough were significantly related to a higher bacillary burden. Our study suggests that Xpert Ultra N-Ct correlates well with smear and TTP, and demonstrates the added value of N-Ct as a predictor of positive smear status.