Abstract
Performance of the Xpert MTB/RIF assay, designed to simultaneously detect Mycobacterium tuberculosis complex (MTBC) and rifampin (RIF) resistance, has been well documented in low-resource settings with high TB-incidence. However, few studies have assessed its accuracy in low TB incidence settings. We evaluated the performance of Xpert MTB/RIF using clinical sputum specimens routinely collected from suspect pulmonary TB patients over a 4-year time period in San Diego County, California. Xpert MTB/RIF results were compared to acid-fast bacilli (AFB) smear microscopy, mycobacterial culture, and phenotypic drug susceptibility testing (DST). Of 751 sputum specimens, 134 (17.8%) were MTBC culture-positive and 2 (1.5%) were multidrug-resistant (MDR). For the detection of MTBC, Xpert MTB/RIF sensitivity was 89.6% (97.7% and 74.5% in smear-positive and -negative sputa, respectively) and specificity was 97.2%; while AFB smear sensitivity and specificity were 64.9% and 77.8%, respectively. Xpert MTB/RIF detected 35 of 47 smear-negative culture-positive specimens, and excluded 124 of 137 smear-positive culture-negative specimens. Xpert MTB/RIF also correctly excluded 99.2% (121/122) of nontuberculous mycobacteria (NTM) specimens, including all 33 NTM false-positives by smear microscopy. For the detection of RIF resistance, Xpert MTB/RIF sensitivity and specificity were 100% and 98.3%, respectively. Our findings demonstrate that Xpert MTB/RIF is able to accurately detect MTBC and RIF resistance in routinely collected respiratory specimens in a low TB-incidence setting, with comparable performance to that achieved in high-incidence settings; and suggest that under these conditions the assay has particular utility in detecting smear-negative TB cases, excluding smear-positive patients without MTBC disease, and differentiating MTBC from NTM.
Highlights
Despite declining global incidence and mortality, tuberculosis (TB) remains a leading cause of death among infectious diseases worldwide, with an estimated 10.4 million new cases and 1.4 million deaths in 2015 [1,2]
Our aim was to evaluate the accuracy of the Xpert MTB/RIF assay in the diagnosis of pulmonary TB and RIF resistance in a low TB-incidence setting, as applied in the course of routine clinical practice
One hundred thirty-four (17.8%) specimens were Mycobacterium tuberculosis complex (MTBC) culture-positive, including M. tuberculosis (n = 132) and M. bovis (n = 2); while 495 (65.9%) specimens were culture-negative for mycobacteria, and 122 (16.3%) yielded nontuberculous mycobacteria (NTM) on culture
Summary
Despite declining global incidence and mortality, tuberculosis (TB) remains a leading cause of death among infectious diseases worldwide, with an estimated 10.4 million new cases and 1.4 million deaths in 2015 [1,2]. In the clinical management of pulmonary TB, rapid and accurate detection of Mycobacterium tuberculosis complex (MTBC) and drug-resistance is essential. Likewise on a population scale, early case detection and appropriate treatment are the most effective control strategies to reduce TB transmission [2,3]. Smear microscopy is unable to distinguish MTBC from nontuberculous mycobacteria (NTM), or viable from nonviable organisms, and has the potential for false-positives and low positive predictive value (PPV). Smear microscopy is unable to differentiate drug resistant and susceptible strains of MTBC [6,7]
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