Molecular methods in the detection and identification of mycobacterial infections

Nucleic acid amplification (NAA) tests for direct detection of Mycobacterium tuberculosis complex in respiratory specimens have the potential to provide a more rapid diagnosis of pulmonary tuberculosis (TB) than is currently possible by conventional stain, culture, and identification tests. Currently, 2 NAA tests-enhanced Amplified Mycobacterium Tuberculosis Direct (MTD) Test (Gen-Probe, Inc) and Amplicor Mycobacterium tuberculosis Test (Roche Molecular Systems, Inc)-have been approved by the Food and Drug Administration for testing respiratory specimens that are smear positive for acid-fast bacilli (AFB). This restriction to AFB smear-positive specimens was based on data from the initial clinical trials conducted to evaluate these products that showed low sensitivity (ie, 48%-53%) and less-than-optimal specificity (ie, 96%-99%) in AFB smear-negative specimens. Data from the clinical trial for the enhanced MTD test and from 2 subsequent studies, however, suggest that this version of the MTD test is a reliable tool for rapid diagnosis of pulmonary TB, regardless of the AFB smear result. Both NAA tests have been evaluated for diagnosis of extrapulmonary TB, and results were comparable to the results of tests performed with respiratory specimens. The NAA tests also appear to be reliable for rapid identification of M tuberculosis complex in positive broth cultures of all specimen types except blood. The impact of the NAA tests on patient outcome varies based on the AFB smear result. With smear-positive results, public health and hospital infection control resources are predominantly affected. With smear-negative results, however, the potential for affecting patient outcome is much greater. In patients with smear-negative results, the NAA test can result in earlier diagnosis of TB and subsequent initiation of therapy. Use of these tests also may eliminate the need for invasive diagnostic procedures, which are costly and pose an added risk to the patient, and they may allow earlier discharge of hospitalized patients.