Performance of real-time PCR Xpert ®MTB/RIF in diagnosing extrapulmonary tuberculosis

The real time PCR Xpert ® MTB/RIF is fundamental for rapid diagnosis in paucibacillary respiratory samples and for the detection of multidrug-resistant TB cases. This paper aimed to determine its performance on different extrapulmonary samples. We determined sensitivity, specificity, positive and negative predictive value on respiratory and non-respiratory samples collected from January 2010 to June 2014. The protocol for the Xpert ® MTB/RIF PCR suggested by Cepheid was strictly followed for all specimens. In 12257 respiratory samples we observed a sensitivity of 87.1% and a specificity of 99.9%. There were 2818 extrapulmonary specimens, of which 250 were followed by a positive culture for Mycobacterium tuberculosis complex, whereas 72 samples were culture-negative: tuberculosis was clinically confirmed in 71 of them and was excluded for one sample. The sensitivity of the test on urine, pus and CSF samples was 88.2%, 95.6% and 100% respectively. In contrast, the sensitivity of gastric aspirates and biopsies was 81.8% and 83.6% respectively, whereas results of total cavitary fluids were significantly worse than expected (53.7% sensitivity). Our experience shows that Xpert MTB/RIF assay is an accurate, sensitive, and specific test for the rapid detection of pulmonary and extra-pulmonary TB with the only exception of cavitary fluids.