Description

Xpert MTB/RIF Ultra shows improved diagnostic performance compared with predecessor test, Xpert MTB/RIF, in a low TB incidence country
M. Mansfield (a), A.M. McLaughlin (b), E. Roycroft (a,c), J. Keane (b), M.M. Fitzgibbon (a,c), T.R. Rogers (a,c)

a) Department of Clinical Microbiology, Trinity College Dublin, St James’s Hospital, Dublin, Ireland; b) Department of Respiratory Medicine, St. James’s Hospital, Dublin, Ireland; c) Irish Mycobacteria Reference Laboratory, St. James’s Hospital, Dublin, Ireland

Background and Objectives: The aim of this study was to evaluate the performance of the Xpert MTB/RIF Ultra (Ultra) compared with its predecessor test, the Xpert MTB/RIF (Xpert), in the diagnosis of Mycobacterium tuberculosis complex infection in a low TB incidence setting.
Methods: A retrospective analysis was performed on 679 samples received between 2015 and 2018 at the Irish Mycobacteria Reference Laboratory, on which the Xpert had been performed, and on 700 samples, received between 2018 and 2020, on which the Ultra had been performed. Samples were pulmonary (n=820) and extrapulmonary (n= 559) in nature, and a total of 237 samples were culture positive for Mycobacterium tuberculosis complex (55/237 smear negative, 182/237 smear positive).The diagnostic performance, including sensitivity, specificity, positive and negative predicative values and diagnostic accuracy, of the Xpert and Ultra assays was analysed using culture as reference standard.
Results:  The overall sensitivity of the Ultra was 93.3% compared with 84.9% for the Xpert among all culture positive MTC samples, inclusive of both smear positive and smear negative samples (p=0.0417). In smear negative-culture positive specimens, the sensitivity of Ultra was 73.1% versus 37.9% for the Xpert (p= 0.0096). Specificity of both assays was comparable at 94.8% for the Ultra and 95.98% for the Xpert (p= 0.3398). The sensitivity among pulmonary samples was 96% for the Ultra and 90.2% for the Xpert (p=0.1514), with respective specificities of 96.4% and 97.5% (p= 0.4366). The sensitivity among extrapulmonary samples was 86.7% for the Ultra and 72.5% for the Xpert (p=0.1557), with specificities of 92.7% and 93.9% respectively (p = 0.5828). Cycle threshold values of both Xpert and Ultra assays showed a moderate correlation with sample time to culture positivity.
Conclusion: The Ultra has improved performance over the Xpert in the diagnosis of MTC infection in a low TB incidence setting, especially for smear negative-culture positive samples.  The sensitivity of the Ultra was greater than that of the Xpert in both pulmonary and extrapulmonary samples, particularly among smear negative specimens. The specificity of the Ultra was slightly lower than that of the Xpert, however, this did not reach statistical significance in any sample category.