Added Value of Xpert MTB/RIF Ultra for Diagnosis of Pulmonary Tuberculosis in a Low-Prevalence Setting

Background: Xpert MTB/RIF (Xpert) for direct molecular detection of Mycobacterium tuberculosis and rifampicin resistance from clinical specimens has dramatically improved the diagnosis of tuberculosis (TB). Xpert MTB/RIF Ultra (Ultra) is proposed as a substitute of Xpert with increased sensitivity and improved rifampicin resistance detection. We evaluated the diagnostic performance of Ultra and Xpert for pulmonary TB diagnosis in a low TB burden setting. Methods: Performance of Ultra and Xpert were compared to culture on respiratory specimens from patients with suspected pulmonary TB (November 2016 to August 2018) (n=196) in Lausanne (Switzerland). Clinical data were used to investigate discrepant results. Correlation between semi-quantitative result of Ultra and smear microscopy status for the detection of acid-fast bacilli (AFB) was established. Results : The sensitivity of Xpert and Ultra were respectively 82.9% (39/47) and 95.8% (45/47) when considering all culture positive specimens, 100% (23/23) for both assays on smear positive specimens and 66.7% (16/24) versus 91.7% (22/24) on smear negative specimens. Using culture as gold standard, the specificity of Xpert and Ultra were respectively 97.3% (145/149) and 96.64% (144/149). All the patients with Ultra positive with the new category “trace” were diagnosed with active TB based on clinical findings and microbiological culture. Both the semi-quantitative result of Xpert and of Ultra positively correlated with the semi-quantitative result of AFB detection. Conclusion: Our data support an increased sensitivity of Ultra when compared to Xpert in a low prevalence setting. Correlation between the Ultra semi-quantitative result and AFB burden can help at evaluating patient’s transmission potential.