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Journal of Clinical Microbiology, April 1999, p. 1236-1236, Vol. 37, No. 4
0095-1137/99/$04.00+0
LETTERS TO THE EDITOR
Comparison of Nucleic Acid Amplification Tests for Tuberculosis
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LETTER |
Piersimoni et al. have reported their experience in parallel
testing of the Amplified Mycobacterium tuberculosis Direct
Test (AMTDII) (Gen-Probe Inc., San Diego, Calif.) and the Abbott LCx Mycobacterium tuberculosis Assay (LCx) (Abbott Laboratories
Diagnostic Division, Abbott Park, Ill.) with 273 respiratory samples
and 184 nonrespiratory samples (1). Their study provides further useful
statistics on the value of the AMTDII and LCx as diagnostic tools for
tuberculosis (TB).
In general, the results presented by Piersimoni et al. support the
contention that regardless of the manufacturer, commercial amplification tests have almost absolute sensitivity (and
specificity) when applied to microscopy-positive
acid-fastbacillus (M-afb) samples. While results with M-afb
samples will be of undeniable value
particularly in localities
with significant rates of disease due to atypical mycobacteriathe
preferred commercial test is likely to be the one that performs best
with (i) samples that are microscopy negative (M-neg) but which grow
M. tuberculosis on culture (C-MTB) and (ii) samples from
patients who are known to have TB but which are M-neg and negative by
culture (C-neg). Numerous earlier studies, using different assays and a
variety of samples, have suggested widely divergent sensitivities with such material.
Piersimoni et al. acknowledge the need for parallel testing with
various kits, and their study was designed to address this issue.
Nevertheless, the authors seem to have missed the opportunity to
present some valuable comparative data. Before looking at specific examples, it should be noted that the abstract of their article states
that "the level of agreement between AMTDII and LCx assay results was
78.2%." But there is no reference to this statistic in the body of
the paper, and the data (as presented) do not allow the reader to check
this figure. The study found that AMTDII "missed" 4 (10.5%) of 38 samples that were M-neg and C-MTB, whereas LCx missed 17 (44.7%).
Further, in testing 25 samples which were from patients with TB but
which were M-neg and C-neg, both assays missed 11 (44.0%). These
findings beg the question, Were the samples missed by AMTDII also
missed by LCx? The authors' conclusion that AMTDII is significantly
more sensitive than LCx with both respiratory and extrapulmonary
samples is of particular interest to laboratories looking for guidance
in choosing a commercial assay. However, a table showing correlated
results for AMTDII and LCx with the subset of samples that gave
inconsistent results would have been most valuable in assessing the
performance of the individual assays.
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REFERENCE |
| 1.
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Piersimoni, C.,
A. Callegaro,
C. Scarparo,
V. Penati,
D. Nista,
S. Bornigia,
C. Lacchini,
M. Scagnelli,
G. Santini, and G. De Sio.
1998.
Comparative evaluation of the new Gen-Probe Mycobacterium tuberculosis Amplified Direct Test and the semiautomated Abbott LCx Mycobacterium tuberculosis Assay for direct detection of Mycobacterium tuberculosis complex in respiratory and extrapulmonary specimens.
J. Clin. Microbiol.
36:3601-3604[Abstract/Free Full Text].
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David J. Dawson
Queensland Diagnostic and Reference Laboratory for Mycobacterial
Diseases The Prince Charles Hospital Chermside 4032, Australia
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AUTHOR'S REPLY |
Dr. Dawson is entirely right when he says that further information
about the subset of samples giving discrepant results should have been
provided. Here are the required data.
Of the 38 specimens (respiratory and extrapulmonary) belonging to the
category "smear negative, culture positive" (Tables 1 and 3 in
our article), 4 were AMTDII negative and 17 were LCx negative. All the
LCx-negative samples were AMTDII positive, while only one of the four
AMTDII-negative specimens was LCx positive.
Of the 13 specimens (respiratory and extrapulmonary) belonging to the
category "smear and culture negative with a final diagnosis of TB"
(Tables 1 and 3 in our article), 5 were positive and 5 were negative by
both assays, respectively. Of the remaining three specimens, two were
AMTDII positive-LCx negative; for the last sample, the opposite held
true. Our data emphasize that the category of "smear-negative,
culture-positive" samples exhibited the most striking differences
(statistically significant) in sensitivities.
Finally, of the 25 specimens mentioned by Dr. Dawson in his letter, all
were from patients undergoing TB chemotherapy. Since commercial
amplification assays are presently used for diagnostic purposes, not
for the monitoring of therapeutic efficacy, these specimens were
reported but not considered for comparative evaluation.
| | | | |
Claudio Piersimoni
Department of Clinical Microbiology General Hospital Umberto
I°-Torrette Via Conca Ancona, I-60020, Italy
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Journal of Clinical Microbiology, April 1999, p. 1236-1236, Vol. 37, No. 4
0095-1137/99/$04.00+0
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