Sixty-four of 85 (75.3%) smear-negative respiratory (n ‫؍‬ 78) and nonrespiratory (n ‫؍‬ 7) samples with positive cultures of Mycobacterium tuberculosis complex (MTC) were detected by the GeneXpert system using the Xpert MTB/RIF assay (GX). In addition, GX found rpoB mutations in all six of the rifampin-resistant strains detected. The test was negative in 20 culture-negative and 20 nontuberculous culture-positive samples (100% specificity). GX offers high potential for the diagnosis of tuberculosis due to its capacity for direct detection of MTC, its rapidity, and its simplicity.Tuberculosis (TB) remains one of the biggest health problems in developing and industrialized countries and is associated with high rates of morbidity and mortality (19). The emergence and spread of Mycobacterium tuberculosis strains resistant to multiple drugs represent a serious threat to TB control worldwide (18). Early diagnosis of active TB and detection of multidrug-resistant (MDR) strains are essential to interrupt transmission. Traditionally, acid-fast bacillus (AFB) smear microscopy has been the initial method for diagnosis, due to its speed, simplicity, and low cost. However, its low sensitivity (45 to 80% of positive cultures), as well as the fact that a significant percentage (17%) of bacillus transmission is due to smear-negative pulmonary cases, limits the usefulness of this technique (1). On the other hand, due to the intrinsic slow growth of these microorganisms, mycobacterial culture (the gold standard method for TB diagnosis) takes several weeks to provide microbiological confirmation. Therefore, other techniques are needed to reduce turnaround time to diagnosis (13,16).In recent years, direct-detection methodologies, most of them based on nucleic acid amplification, have appeared as potentially useful tools for the rapid diagnosis of TB (7)(8)(9)(11)(12)(13)(14). Guidelines for their use have been defined and recently updated (6).A real-time automated integrated system, the GeneXpert system using the Xpert MTB/RIF assay (Cepheid, Sunnyvale, CA) (GX), has recently been developed and evaluated (2, 3, 10). GX integrates DNA extraction, genomic amplification (heminested PCR), semiquantitative detection of M. tuberculosis complex (MTC), and rifampin (RIF) resistance determination in a single cartridge, thus reducing labor time and crosscontamination risk.The aim of the present study was to evaluate the effectiveness of GX for direct detection of MTC and RIF resistance in smear-negative clinical respiratory and nonrespiratory samples.One hundred twenty-five smear-negative clinical samples from 122 patients, collected over a 10-year period, were retrospectively studied; 85 samples had MTC-positive cultures, 20 samples (all respiratory) had isolates of nontuberculous mycobacteria (5 rapidly growing mycobacteria, 4 Mycobacterium avium-intracellulare complex isolates, 3 Mycobacterium kansasii isolates, and 8 others), and 20 samples had negative mycobacterial cultures (Table 1). Nonsterile clinical samples were pretreated accordi...

Several molecular systems for direct detection of Mycobacterium tuberculosis complex (MTBC) have recently been developed. The GenoQuick MTB assay (GQ-MTB) used in this study detected 82 of the 96 (85.4%) samples with MTBC, including 50 of 64 (78.1%) samples with negative acid-fast bacillus smears. Fifteen samples containing nontuberculous mycobacteria were also studied: 13 were GQ-MTB negative, one was positive, and one was indeterminate. GQ-MTB showed good effectiveness for the direct detection of MTBC from clinical samples.

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