Abstract
BackgroundThe Xpert MTB/RIF assay (Xpert assay; Cepheid, Sunnyvale, CA) is becoming the test of choice for the rapid diagnosis of tuberculosis and rifampin (RIF) resistance. The aim of this study was to evaluate the performance of the Xpert assay with respect to its clinical application at a tertiary care hospital in Korea, a country with an intermediate tuberculosis burden and high-resource.MethodsA total of 303 Xpert assay results from 109 smear-positive and 194 smear-negative respiratory specimens were retrospectively reviewed. Based on patients’ medical records, four categories of clinical applications of the Xpert assay were identified: (1) the diagnosis of pulmonary tuberculosis in patients with a high probability of pulmonary tuberculosis according to their clinical and radiological features; (2) the exclusion of tuberculosis in clinically indeterminate patients for pulmonary tuberculosis; (3) the differentiation of Mycobacterium tuberculsosis (MTB) from nontuberculous mycobacteria in a smear-positive specimen; and (4) the diagnosis of RIF resistance. Standard culture and drug susceptibility tests were used as reference methods.ResultsThe sensitivity of the Xpert assay for MTB detection in category 1 was 89.8% (95% confidence interval [CI], 78.5-95.8%), but 66.7% (95% CI, 12.5-98.2%) in category 2. The positive predictive values ranged from 33.3% (95% CI, 6.0-75.9%) in category 2 to 91.3% and 91.7% in categories 1 and 3, respectively. The negative predictive values were over 90% in all categories. The Xpert assay correctly detected RIF resistance in six of the seven (85.7%) isolates tested.ConclusionsThe Xpert assay exhibited variable performance according to its clinical application; this finding cautions that careful interpretation for the results of this assay would be needed according to its intended purpose.Electronic supplementary materialThe online version of this article (doi:10.1186/s12879-014-0589-x) contains supplementary material, which is available to authorized users.
Highlights
The Xpert Mycobacterium tuberculosis (MTB)/RIF assay (Xpert assay; Cepheid, Sunnyvale, CA) is becoming the test of choice for the rapid diagnosis of tuberculosis and rifampin (RIF) resistance
Category 3 consisted of samples from patients with an acid-fast bacilli (AFB) smear-positive specimen, for which MTB needed to be differentiated from nontuberculous mycobacteria (NTM)
After exclusion of nine nonrespiratory specimens and 86 samples for which cultures were not requested at the same day, a total of 303 respiratory specimens (264 sputum samples and 39 samples of bronchial washing or bronchoalveolar lavage fluid) from 300 patients were used to analyze the diagnostic performance of the assay
Summary
The Xpert MTB/RIF assay (Xpert assay; Cepheid, Sunnyvale, CA) is becoming the test of choice for the rapid diagnosis of tuberculosis and rifampin (RIF) resistance. The Xpert assay can be utilized as a rapid diagnostic test for tuberculosis in patients with presumptive pulmonary tuberculosis, and can be used to rapidly exclude tuberculosis (e.g., for determining airborne infection isolation discontinuation) or to differentiate MTB from nontuberculous mycobacteria (NTM) in smear-positive cases. This assay may be used to determine whether a patient with tuberculosis is infected with a RIF-resistant strain [3,4,5,6]. The performance of the Xpert assay has not yet been assessed in an intermediateincidence, high-resource setting with respect to its different applications [9,10,11]
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