Accuracy of the Xpert® MTB/RIF assay for the diagnosis of extra-pulmonary tuberculosis in South Korea

Abstract

Setting: The Xpert(®) MTB/RIF assay has been endorsed by the World Health Organization for the detection of pulmonary and extra-pulmonary tuberculosis (EPTB).

Objective: To determine the accuracy of the Xpert assay in diagnosing EPTB in South Korea, a country with an intermediate TB burden.

Design: We retrospectively reviewed the medical records of 1429 patients in whom the Xpert assay using EPTB specimens was requested between 1 January 2011 and 31 October 2013 in a tertiary referral hospital in South Korea. The sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) for the diagnosis of EPTB and detection of rifampicin (RMP) resistance were calculated.

Results: Using culture as gold standard, the sensitivity, specificity, PPV and NPV of the assay were respectively 67.7%, 98.1%, 60% and 98.6%. Using a composite reference standard, the sensitivity, specificity, PPV and NPV were respectively 49.3%, 100%, 100% and 95.1%. The sensitivity, specificity, PPV and NPV for the detection of RMP resistance among specimens with positive results for Mycobacterium tuberculosis were respectively 80%, 100%, 100% and 97.7%.

Conclusion: The Xpert assay showed acceptable sensitivity in certain groups and excellent specificity in diagnosing EPTB and detecting RMP resistance in an intermediate TB burden country.