The transition to Xpert MTB/RIF ultra: diagnostic accuracy for pulmonary tuberculosis in Kampala, Uganda

BackgroundThe World Health Organization (WHO) has endorsed the next-generation Xpert MTB/RIF Ultra (Ultra) cartridge, and Uganda is currently transitioning from the older generation Xpert MTB/RIF (Xpert) cartridge to Ultra as the initial diagnostic test for pulmonary tuberculosis (TB). We assessed the diagnostic accuracy of Ultra for pulmonary TB among adults in Kampala, Uganda.MethodsWe sampled adults referred for Xpert testing at two hospitals and a health center over a 12-month period. We enrolled adults with positive Xpert and a random 1:1 sample with negative Xpert results. Expectorated sputum was collected for Ultra, and for solid and liquid culture testing for Xpert-negative patients. We measured sensitivity and specificity of Ultra overall and by HIV status, prior history of TB, and hospitalization, in reference to Xpert and culture results. We also assessed how classification of results in the new trace category affects Ultra accuracy.ResultsAmong 698 participants included, 211 (30%) were HIV-positive and 336 (48%) had TB. The sensitivity of Ultra was 90.5% (95% CI 86.8-93.4) and specificity was 98.1% (95% CI 96.1-99.2). There were no significant differences in sensitivity and specificity by HIV status, prior history of TB or hospitalization. Xpert and Ultra results were concordant in 670 (96%) participants, with Ultra having a small reduction in specificity (difference 1.9, 95% CI 0.2 to 3.6, p=0.01). When trace results were considered positive for all patients, sensitivity increased by 2.1% (95% CI 0.3 to 3.9, p=0.01) without a significant reduction in specificity (-0.8, 95% CI -0.3 to 2.0, p=0.08).ConclusionsAfter 1 year of implementation, Ultra had similar performance to Xpert. Considering trace results to be positive in all patients increased case detection without significant loss of specificity. Longitudinal studies are needed to compare the benefit of greater diagnoses to the cost of overtreatment.

Automated summary

In Kampala, Uganda, the evaluation of Ultra cartridge for pulmonary tuberculosis diagnosis among adults showed a sensitivity of 90.5% and specificity of 98.1%, with no significant differences compared to Xpert. Considering trace results as positive increased case detection without significant loss of specificity.