Vol. 41, No. 6
Interpreting the Results of the Ampliﬁed Mycobacterium tuberculosis Direct Test for Detection of M. tuberculosis rRNA
Andrew M. Middleton,1 Paul Cullinan,2 Robert Wilson,3 Jonathan R.Kerr,1 and Maureen V. Chadwick1*
Department of Microbiology1 and Host Defence Unit,3 Royal Brompton and Hareﬁeld Hospitals NHS Trust, and Department of Occupational and Environmental Medicine, Imperial College of Science, Technology and Medicine,2 London, United Kingdom
Received 29 July 2002/Returned for modiﬁcation 21 January 2003/Accepted 4 March 2003
The Ampliﬁed Mycobacterium tuberculosisDirect (AMTD) test detects M. tuberculosis rRNA. By using culture of M. tuberculosis as a gold standard, a number of different diagnostic indices were examined in an attempt to determine the diagnostic performance of the AMTD test and demonstrate how it might usefully be interpreted during the early management of disease. Recent developments in diagnostic methods have decreased the time needed fordetection and identiﬁcation of Mycobacterium tuberculosis, but the process still requires at least 2 weeks. The Ampliﬁed Mycobacterium tuberculosis Direct test (AMTD test; Gen-Probe, Inc., San Diego, Calif.) can be used to detect M. tuberculosis rRNA in respiratory specimens (sputum, broncho-alveolar lavage [BAL], and bronchial and tracheal secretions) and can be performed in approximately 3.5 h.With this test speciﬁc RNA ampliﬁcation products are hybridized to complementary acridinium-ester-labeled DNA probes. Subsequent degradation of the acridinium-ester probes results in luminescence that is measured in relative light units (RLU). According to the manufacturers an RLU reading of 30,000 signiﬁes a positive test, but they recommend repeating the tests at between 30,000 and 100,000 RLU.Sensitivities and speciﬁcities of the test at other RLU values, in comparison with a clinical diagnosis of tuberculosis, have been reported (1) with results of around 90% for each. Here we report the performance of the AMTD test with a consecutive series of hospital patients suspected of tuberculous infection and demonstrate how the results might usefully be interpreted during their earlymanagement. From 1994 to 2001 434 sputum and 339 BAL samples from separate patients were received for AMTD testing and mycobacterial culture. All were processed by a modiﬁed Petroff’s method (3) prior to AMTD testing and culture on Lowenstein¨ Jensen slopes and in Middlebrook 7H12 radiometric broth (Becton Dickinson, Sparks, Md.). Cultured M. tuberculosis was identiﬁed by biochemical tests (5) and/orAccu-Probe culture conﬁrmation tests (Gen-Probe, Inc.). Of the 773 respiratory samples, 178 were smear positive (146 sputum, 32 BAL). Ninety-two cultures grew M. tuberculosis, 66 from sputa (54 smear positive, 12 smear negative) and 26 from BAL (12 smear positive, 14 smear negative). Seventy-ﬁve cultures (10%) grew nontuberculous mycobacteria. The median
* Corresponding author. Mailing address:Department of Microbiology, Royal Brompton and Hareﬁeld Hospitals NHS Trust, Sydney St., London SW3 6NP, United Kingdom. Phone: 44-207-3518446. Fax: 44-207-3518443. E-mail: email@example.com. 2741
RLU value for specimens with positive cultures for M. tuberculosis was 2,345,744 (range, 3,094 to 3,648,998 RLU), and for those with negative cultures (including nontuberculous mycobacteria)the RLU value was 7,031 (range, 448 to 3,609,521 RLU). The median RLU value for specimens with only positive cultures for nontuberculous mycobacteria was 12,870 (range, 1,205 to 3,424,688). Figure 1 shows the distribution of RLU readings for AMTD tests on samples with negative cultures or with cultures of either M. tuberculosis or nontuberculous mycobacteria. Using the culture results as the “gold...