Although culture remains the gold standard for laboratory confirmation of TB, the CDC now recommends that nucleic acid amplification (NAA) testing become standard practice in helping to diagnose people in whom pulmonary TB is moderately to highly suspected (MMWR. 2009;58:7-10).
The NAA test has been commercially available in the United States since the late 1990s. It can deliver results within 24-48 hours, as opposed to the two to six weeks required for culture. Recent studies have shown that clinicians already rely on the NAA test result as the deciding factor for the initiation of therapy for 20%-50% of TB cases in settings where NAA testing is a routine practice. However, the test should not be ordered if the suspicion of TB is low, as its positive predictive value in such cases is <50%. On the other hand, the CDC cautions that a single negative NAA test result should not be used as a definitive result to exclude TB, particularly when the illness is strongly suspected.