Evaluation of Xpert MTB/RIF Ultra performance for pulmonary tuberculosis diagnosis on smear-negative respiratory samples in a French centre

Tuberculosis (TB) is a worldwide public health concern, including in high-resource countries with a low prevalence of TB. Xpert MTB/RIF assay was developed to improve TB and rifampicin (RIF) resistance detection, but sensitivity remains poor on smear-negative sputum. Xpert MTB/RIF Ultra assay was designed to enhance the sensitivity of TB detection in clinical samples. Herein, we evaluated retrospectively the performance of this test on smear-negative respiratory samples. Respiratory specimens with smear-negative and a Mycobacterium tuberculosis (MTB) complex-positive culture were retrospectively selected from those taken from patients during routine care, and analysed in the Mycobacteria Laboratory of the Lyon University hospital, France. Specimens were stored at - 20 °C before testing by Xpert MTB/RIF Ultra. For each sample, growth delay and date of anti-TB treatment initiation were recorded. Forty-six samples-29 sputum, 8 bronchial aspirates, 6 broncho-alveolar lavages, and 3 gastric aspirates-were selected. Among samples collected before treatment initiation (n = 33), sensitivity was 81.8% (95% CI [64.5; 93.0]) and there was a significant correlation between the quantitative measurements (Ct) of Xpert MTB/RIF Ultra assay and the time to growth detection in culture. Among samples collected after treatment initiation (n = 12), sensitivity was 100%, without correlation with time to growth detection due to presence of afterglow DNA in samples. In high-resource settings, the Xpert MTB/RIF Ultra test represents a useful tool for pulmonary TB diagnosis, notably for the paucibacillary forms. Moreover, quantitative measurement of Xpert MTB/RIF Ultra could help to predict time to MTB culture positivity and be used as a quality indicator of MTB culture process.