False-positive gen-probe direct Mycobacterium tuberculosis amplification test results for patients with pulmonary M-kansasii and M-avium infections

The Gen-Probe Amplified Mycobacterium Tuberculosis Direct (MTD) test has be en approved for use in the United States for the rapid diagnosis of pulmona ry tuberculosis in patients with acid-fast smear-positive sputum samples si nce 1996. Four patients infected with human immunodeficiency virus and one chronic pulmonary-disease patient seen in our institutions with abnormal ch est radiographs and fluorochrome stain-positive sputa were evaluated for tu berculosis, including performance of the MTD test on expectorated sputum sa mples. Three of these five patients' sputa were highly smear-positive (i.e. , more than 100 bacilli per high-power field), while two patient's sputa co ntained 1 to 10 bacilli per field. MTD results on sputum specimens from the se patients ranged from 43,498 to 193,858 relative light units (RLU), Gen-P robe has defined values of at least 30,000 RLU as indicative of a positive test, i.e., the presence of Mycobacterium tuberculosis RNA. Four of the pat ients' sputum cultures yielded growth of M. kansasii within 6 to 12 days, a nd the fifth produced growth of M. avium only. One patient's culture contai ned both M, kansasii and M, avium, but none of the initial or follow-up cul tures from these five patients revealed M, tuberculosis. However, subsequen t cultures from three of the patients again revealed M, kansasii, During th e period of this study, in which MTD tests were performed on smear-positive sputum specimens from 82 patients, four of seven patients with culture-pro ven M. kansasii pulmonary infections yielded one or more false-positive MTD tests. The MTD sensitivity observed in this study was 93.8%, and the speci ficity was 85.3%. Five cultures of M. kansasii (including three of these pa tients' isolates and Ill kansasii ATCC 12478), and cultures of several othe r species were examined at densities of 10(5) to 10(7) viable CFU/ml by the MTD test. All five isolates of M. kansasii and three of three isolates of M, simiae yielded false-positive test results, with readings of 75,191 to 3 35,591 RLU, These findings indicate that low-level false-positive MTD resul ts can occur due to the presence of M. kansasii, Ill. avium, and possibly o ther Mycobacterium species other than M, tuberculosis in sputum, Low-level positive MTD results of 30,000 to 500,000 RLU should be interpreted in ligh t of these findings. It remains to be determined if the enhanced MTD test ( MTD 2) recently released by Gen-Probe will provide greater specificity than that observed in this report with its first-generation test.