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Journal of Clinical Microbiology, January 1999, p. 175-178, Vol. 37, No. 1
Departments of
Pathology1 and
Medicine,2 The University of Texas
Health Science Center, San Antonio, Texas 78284;
Microbiology
Laboratory, University Hospital, San Antonio, Texas
782293; and
Baptist Health System,
San Antonio, Texas 782054
Received 16 July 1998/Returned for modification 22 September
1998/Accepted 6 October 1998
The Gen-Probe Amplified Mycobacterium Tuberculosis Direct (MTD)
test has been approved for use in the United States for the rapid
diagnosis of pulmonary tuberculosis in patients with acid-fast smear-positive sputum samples since 1996. Four patients infected with
human immunodeficiency virus and one chronic pulmonary-disease patient
seen in our institutions with abnormal chest radiographs and
fluorochrome stain-positive sputa were evaluated for tuberculosis, including performance of the MTD test on expectorated sputum samples. 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
contained 1 to 10 bacilli per field. MTD results on sputum specimens
from these patients ranged from 43,498 to 193,858 relative light units
(RLU). Gen-Probe 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 patients' sputum cultures yielded growth of M. kansasii within 6 to
12 days, and the fifth produced growth of M. avium only.
One patient's culture contained both M. kansasii and
M. avium, but none of the initial or follow-up cultures
from these five patients revealed M. tuberculosis. However,
subsequent cultures from three of the patients again revealed M. kansasii. During the period of this study, in which MTD tests
were performed on smear-positive sputum specimens from 82 patients,
four of seven patients with culture-proven M. kansasii
pulmonary infections yielded one or more false-positive MTD tests. The
MTD sensitivity observed in this study was 93.8%, and the specificity
was 85.3%. Five cultures of M. kansasii (including three
of these patients' isolates and M. kansasii ATCC 12478), and cultures of several other species were examined at densities of
105 to 107 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 335,591 RLU. These findings indicate that
low-level false-positive MTD results can occur due to the presence of
M. kansasii, M. avium, and possibly other
Mycobacterium species other than M. tuberculosis in sputum. Low-level positive MTD results of 30,000 to 500,000 RLU should be interpreted in light 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.
0095-1137/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
False-Positive Gen-Probe Direct Mycobacterium
tuberculosis Amplification Test Results for Patients with
Pulmonary M. kansasii and M. avium
Infections
*
Corresponding author. Mailing address: Department of
Pathology, University of Texas Health Science Center, 7703 Floyd Curl Dr., San Antonio, TX 78284-7750. Phone: (210) 567-4088. Fax: (210) 567-2367. E-mail: jorgensen{at}uthscsa.edu.
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