Evaluation of the Xpert MTB/RIF assay for the diagnosis of pulmonary tuberculosis in a high HIV prevalence setting

Abstract

Rationale: Xpert MTB/RIF is a novel automated molecular diagnostic recently endorsed by the World Health Organization. However, performance-related data from high HIV prevalence settings are limited.

Objectives: The impact of sample-related factors on performance and the significance of Xpert MTB/RIF-positive culture-negative discordance remain unclear.

Methods: Xpert MTB/RIF was evaluated using single archived spot-sputum samples from 496 South African patients with suspected TB. Mycobacterium tuberculosis culture positivity and phenotypic resistance to rifampicin served as reference standards.

Measurements and main results: Overall, Xpert MTB/RIF detected 95% (95% confidence interval [CI], 88-98%; 89 of 94) of smear-positive culture-positive cases and the specificity was 94% (91-96%; 320 of 339). The sensitivity in smear-negative cases was 55% (35-73%; 12 of 22) when the analysis was restricted to 1 ml of unprocessed sputum and culture time-to-positivity of less than or equal to 28 days. Compared with smear microscopy (n=94), Xpert MTB/RIF detected an additional 17 cases (n=111) representing an 18% (11-27%; 111 vs. 94) relative increase in the rapid TB case detection rate. Moreover, compared with smear microscopy, the inclusion of Xpert MTB/RIF-positive culture-negative TB cases (ruled-in by an alternative diagnostic method) resulted in the detection of a further 16 cases (n=127), thus significantly increasing the rapid TB case detection rate to 35% (95% CI, 26-45%; 94 to 111 vs. 94 to 127; P<0.01), the overall specificity to 99.1% (97-100%; 320 of 323; P<0.001), and sensitivity in smear-negative TB to 60% (P=0.12). Performance strongly correlated with smear status and culture time-to-positivity. In patients infected with HIV compared with patients uninfected with HIV Xpert MTB/RIF showed a trend to reduced sensitivity (P=0.09) and significantly reduced negative predictive value (P=0.01). The negative predictive value for rifampicin resistance was 99.4%.

Conclusions: XpertMTB/RIF outperformed smear microscopy, established a diagnosis in a significant proportion of patients with smear-negative TB, detected many highly likely TB cases missed by culture, and accurately ruled out rifampicin-resistant TB. Sample-specific factors had limited impact on performance. Performance in patients infected with HIV, especially those with advanced immunosuppression, warrants further study.