Xpert MTB/RIF Ultra for the rapid diagnosis of extrapulmonary tuberculosis in a clinical setting of high tuberculosis prevalence country and interpretation of 'trace' results

Abstract

To evaluate the diagnostic performance of Xpert MTB/RIF Ultra (Ultra) for the diagnosis of extrapulmonary tuberculosis (EPTB) from different types of extrapulmonary specimens in comparison with culture and composite microbiological reference standard (CRS). A total of 240 specimens were prospectively collected from presumptive EPTB patients between July 2021-January 2022 and tested by Ultra, Xpert, culture and acid-fast bacilli (AFB) smear microscopy. Out of 240 specimens, 35.8 %, 20.8 %, 11.3 %, and 7.1 % were detected as Mycobacterium tuberculosis complex by Ultra, Xpert, culture and AFB microscopy, respectively. An additional 15.0 % cases were detected by Ultra compared to Xpert MTB/RIF (Xpert) assay. A total of 28 (11.7 %) cases were identified as 'trace' category by Ultra with indeterminate rifampicin resistance result; of which 36.4 % were clinically confirmed as EPTB. Compared to culture, the sensitivity and specificity of Ultra and Xpert were 100 % and 72.3 %; 92.6 % and 88.3 %, respectively. In comparison with CRS, these were respectively: 98.9 % and 100 %; 57.5 % and 100 %. For individual category of specimens, sensitivity of Ultra was 100 % with varying specificity. We found that Ultra was highly sensitive for the rapid diagnosis of EPTB and has extensive potential over current diagnostics in high TB burden countries, but 'trace' results should be interpreted with caution.

Keywords: Extrapulmonary tuberculosis; Mycobacterium tuberculosis; Trace call; Xpert MTB/RIF assay; Xpert MTB/RIF ultra.