Performance of Xpert MTB/RIF Ultra assay for diagnosis of tuberculosis: A retrospective study in a city with intermediate tuberculosis burden

Xpert MTB/RIF Ultra assay (Ultra) was shown to be useful for diagnosis of tuberculosis (TB) in areas of high and low TB burden. However, data from intermediate TB burden settings remains limited. This study aims to evaluate the performance of Ultra in Hong Kong, China, a city with intermediate TB burden.

Consecutive adult patients in Haven of Hope Hospital with samples processed with microscopy, Ultra and mycobacterial culture during routine practice between 1st December 2018 and 31st December 2020 were identified retrospectively. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of Ultra were calculated, using mycobacterial culture as the gold standard and a composite reference standard . Test performance in detecting rifampicin resistance was evaluated.

A total of 1706 samples (1556 respiratory, 150 extra-pulmonary) were included, with 269 culture-positive TB, and 345 clinical TB. Overall, Ultra had a sensitivity of 96.7% (95% confidence interval [CI]: 94.0%–98.4%), specificity of 96.7% (95% CI: 95.6%–97.5%), PPV of 84.4% (95% CI: 80.1%–88.2%) and NPV of 99.4% (95% CI: 98.8%–99.7%) for the detection of tuberculosis against mycobacterial culture. Against composite reference standard as a reference standard, sensitivity was lowered to 84.3% (95% CI: 80.3%–87.9%), but specificity was higher at 98.8% (95% CI: 98.1%–99.3%). Sensitivity, specificity, PPV and NPV of Ultra to detect rifampicin resistance were 100.0% in this study.

Ultra is highly sensitive and specific in diagnosing pulmonary tuberculosis and useful in diagnosing extra-pulmonary tuberculosis in a population with intermediate tuberculosis burden. It also has good performance in detecting rifampicin resistance.