Diagnostic accuracy of Truenat MTB Ultima on sputum for pulmonary tuberculosis diagnosis in an HIV-endemic setting

Objectives Assess Truenat MTB Ultima (Ultima) diagnostic accuracy, alongside standard-of-care Truenat MTB Plus (MTB Plus) and Xpert MTB/RIF Ultra (Ultra) tests, for pulmonary tuberculosis (TB) diagnosis in a high-burden setting. We secondarily evaluated Truenat MTB-RIF Dx (MTB-RIF Dx) on Ultima- or MTB Plus-positive samples for rifampicin susceptibility diagnosis. Methods Adults (≥18 years; n = 498) with presumptive TB self-presenting to primary care clinics in Cape Town, South Africa (19/02/2016-22/02/2023) provided sputa. The microbiological reference standard was a single culture for TB and MTBDRplus on an isolate for rifampicin susceptibility. Results In total, 54% (n = 269) of the participants had HIV, and 42% (n = 210) had previous TB. The proportion of Ultima and MTB Plus unsuccessful results was 14% (95% CI 11, 16) and 20% (17, 23), respectively, with at least half resolving upon retesting the same eluate. In a three-way analysis, Ultima, MTB Plus, and Ultra had TB sensitivities of 90% (85, 93), 84% (78, 88), and 92% (87, 95) and specificities of 85% (80, 88), 95% (92, 97), and 95% (92, 97). Ultima specificity did not improve with Ultra in the reference standard. MTB-RIF Dx had high unsuccessful result rates that varied if done on the day of DNA extraction or on Ultima- [18% (10, 26) vs. 44% (35, 51) if after day of extraction] or MTB Plus-positive eluates [9% (3, 16) vs. 27% (18, 35)]. Same day rifampicin susceptibility testing was often unsuccessful in samples with the "very low" semiquantitation category reported by Ultima [75% (65, 86)] or MTB Plus [73% (58, 89)] but had 100% (40, 100) sensitivity and 99% (96, 100) specificity (on both MTB Plus- or Ultima-positive DNA). Reagent lot variation in unsuccessful and false positive results was observed. Discussion Ultima met the minimum sensitivity recommended by the WHO for TB detection, but specificity, reagent lot variation, and unsuccessful results were suboptimal.