Recent technological developments have introduced a new method to identifying M. tuberculosis complex DNA in clinical samples directly. The direct amplification test (DAT) is approved for identifying M. tuberculosis complex in respiratory specimens that are smear-positive for acid-fast bacilli (AFB). When there is a discrepancy between the AFB smear and DAT, no information on their clinical utility is currently available. In this study, the diagnostic reliability of DAT was investigated in suspected pulmonary tuberculosis patients whose sputum AFB smear was negative.

From June 1, 1998 through May 30, 1999, 909 patients with presumed active pulmonary tuberculosis were enrolled. A sputum AFB stain, culture, DAT and /or biopsy were performed. using the criteria of clinical tuberculosis or confirmed tuberculosis, the positive predictive value of DAT in diagnosing pulmonary tuberculosis was investigated.

The positive predictive value of DAT was 82.1% by the clinically active tuberculosis criteria. However, it decreased to 61.5% when diagnosis was restricted to only to culture positive or biopsy proven cases. The false positive rate of DAT was 18.0%.

The DAT is a valuable diagnostic method in suspected patients whose sputum AFB is was negative.