Xpert Ultra Assay on Stool to Diagnose Pulmonary Tuberculosis in Children

Abstract

Background World Health Organization recommends Xpert MTB/RIF Ultra assay for diagnosing pulmonary tuberculosis (PTB) in children. Though stool is a potential alternative to respiratory specimens among children, the diagnostic performance of Xpert Ultra on stool is unknown. Thus, we assessed the diagnostic performance of Xpert Ultra on stool to diagnose PTB in children. Methods We conducted a cross-sectional study among consecutively recruited children (<15 years) with presumptive PTB admitted in four tertiary care hospitals of Dhaka, Bangladesh between January-2018 and April-2019. Single induced sputum and stool specimens were subjected to culture, Xpert and Xpert Ultra. We considered children as ‘bacteriologically confirmed on induced sputum’ if any tests performed on induced sputum was positive for Mycobacterium tuberculosis and ‘bacteriologically confirmed’ if Mycobacterium tuberculosis was detected on either induced sputum or stool. Results Of 447 children, 29 (6.5%) were ‘bacteriologically confirmed on induced sputum’ and 72 (16.1%) were ‘bacteriologically confirmed’. With ‘bacteriologically confirmed on induced sputum’ as a reference, the sensitivity and specificity of Xpert Ultra on stool was 58.6% and 88.1%, respectively. Whereas, Xpert on stool had sensitivity and specificity of 37.9% and 100.0%, respectively. Among ‘bacteriologically confirmed’, Xpert Ultra on stool was positive in 60 (83.3%) children, of which 48 (80.0%) had ‘trace call’. Conclusions In children, Xpert Ultra on stool has better sensitivity but lesser specificity than Xpert. A high proportion of Xpert Ultra positive on stool had ‘trace call’. Future longitudinal studies on clinical evolution are required to suggest on the management of children with ‘trace call’.

Xpert MTB/RIF Ultra assay for the detection of Mycobacterium tuberculosis in people with negative conventional Xpert MTB/RIF but chest imaging suggestive of tuberculosis in Dhaka, Bangladesh

“…For calculation of sensitivity and specificity of Ultra at 95% confidence interval (CI) using bacteriological-positive PTB by any other method (smear microscopy, culture or Xpert) as reference, we took four different approaches for interpretation of Ultra-trace: all Ultra-trace as Ultra-positive; all Ultra-trace as Ultra-negative; conditional trace where Ultra-trace were categorized as Ultra-positive on the condition that the patient has no history of TB, as those PTB presumptives who had a recent TB episode (within last two years) could have remnants of dead bacilli in their sputum; and Ultra-trace as Ultra-positive only when retesting of the same specimen indicates Ultra-trace or any categorical Ultra-positive, irrespective of history of TB (World Health Organization, 2017). To increase reliability of the evaluation of test performances, composite bacteriological-positive reference using smear, culture and/or Xpert was opted for this study since (i) majority of study population were Xpert-negative while some were 'Xpert-positive and culture-negatives', indicating paucibacillary disease, (ii) use of solid culture which has relatively lower yield than liquid culture and, (iii) culture remains 'imperfect reference' as its sensitivity rate is only about 77% for TB diagnosis (Figure 1) (Kidenya et al, 2013, Opota et al, 2019a, Theron et al, 2018, Zijenah, 2018, Kabir et al, 2021, Nicol et al, 2018. We calculated the inter-test reliability (Cohen's kappa, ꓗ) for determining concordance of Ultra with bacteriological-positive and estimated the increased yield by Ultra (Ho et al, 2019).…”

“…As per the manufacturer, Ultra detects lower concentrations (15 colony forming units (CFU)/ml) of Mycobacterium tuberculosis (MTB) which approximates the limit of detection of MTB in liquid culture, a fold improvement over that of Xpert (112 CFU/ml) (Berhanu et al, 2018). It is expected that Ultra's higher sensitivity will allow for improved diagnosis of paucibacillary forms of TB such as those occuring in young children, certain forms of extra-pulmonary, or in HIV-associated disease (Berhanu et al, 2018, García-Basteiro et al, 2017, Kabir et al, 2021, Opota et al, 2019b, Zijenah, 2018. However, given the lower level of MTB DNA detection by Ultra, particularly with 'trace-calls' where it has been suggested to repeat the test, there remains concern about false-positive results.…”

Diagnostic accuracy of Mycobacterium tuberculosis cell-free DNA for tuberculosis: A systematic review and meta-analysis

“…Although it may be possible to obtain BALF via fiberoptic bronchoscopy, this invasive procedure may be poorly tolerated by patients with milder disease. The use of non-respiratory specimens such as gastric or stool specimens to diagnose PTB has also been reported but remains controversial [ 41 , 42 ]. Plasma-based cfDNA assay testing offers a new pathway for PTB diagnosis.…”

Xpert® MTB/RIF assay testing on stool for the diagnosis of paediatric pulmonary TB in Tanzania

“…27 Using a simple stool testing method, we were able to identify TB in 62% of children and was negative in all children not confirmed with TB. While results with stool testing on Xpert assay, and more recently, with Xpert ® MTB/RIF Ultra (Cepheid) assay 28 in children have been promising, not enough evidence exists to replace testing of respiratory samples rather the need to offer complementary testing with stool.…”