Baseline Xpert MTB/RIF Ultra Cycle Threshold provides a good estimate of bacterial load and has added value as a predictor of smear positivity in patients with pulmonary tuberculosis

Abstract 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.