Diagnostic performance of Xpert MTB/RIF Ultra assay with pulmonary and extrapulmonary specimens: a retrospective evaluation in a low-incidence setting in Finland.

Objective: The aim was to evaluate the sensitivity and specificity of the Xpert MTB/RIF Ultra (Xpert Ultra) assay in the detection of extrapulmonary tuberculosis (TB) in comparison to pulmonary TB in a low-incidence setting in the Helsinki capital area of Finland.

Methods: The retrospective analysis included results from 1112 pulmonary and 705 extrapulmonary samples collected between 2018 and 2023, of which 193 and 136 were culture-positive for Mycobacterium tuberculosis (MTB), respectively. Xpert Ultra results were compared with mycobacterial culture. PCR-positive and culture-negative cases were separately compared with available clinical data (composite reference standard, CRS).

Results: Compared with culture, Xpert Ultra demonstrated 95.3% sensitivity (95% CI, 91.3%-97.7%) and 94.5% specificity (95% CI, 92.8%-95.8%) with pulmonary samples, 47.1% (95% CI, 26.2%-69.0%) and 96.7% (95% CI, 93.8%-98.4%) with pleural fluid, 100% (95% CI, 86.9%-100%) and 81.8% (95% CI, 72.4%-88.6%) with tissue, 96.6% (95% CI, 81.4%-100%) and 75.0% (95% CI, 62.2%-84.6%) with pus, and 95.1% (95% CI, 83.0%-99.5%) and 67.5% (95% CI, 51.9%-80.0%) with lymph node samples, respectively. Other, less common sample types were also included. When CRS was also considered, specificity exceeded 93% for all sample types. Sensitivity was 100% with both smear-positive pulmonary and smear-positive extrapulmonary samples. Neither false rifampicin susceptibility testing results nor cross-reactivity with nontuberculous mycobacteria was detected.

Discussion: Xpert Ultra detected MTB in lymph node, tissue, and pus samples with high-accuracy comparable with the analysis of pulmonary samples while reducing the time to diagnosis by up to several weeks compared with mycobacterial culture.