Faster, More Accurate and Cheaper Tuberculosis Test
Developed
New procedure should have major impact in developing
countries
By Kenna Lowe School of Public Health
Researchers from the Johns Hopkins
Bloomberg School of
Public Health, the Imperial College London and
Universidad Peruana Cayetano Heredia, in Lima, Peru, and
other institutions have developed a simple and rapid new
tuberculosis test. The test, called microscopic-observation
drug-susceptibility, or MODS, is more sensitive, faster and
cheaper to perform than current culture-based tests. The
study is published in the Oct. 12 issue of the New
England Journal of Medicine.
"In the fight to stop the spread of TB, new diagnostic
tools are urgently needed to detect TB and
multidrug-resistant TB. MODS is just such a tool. It will
change the practice of TB testing in developing countries,"
said Robert H. Gilman, senior author of the study and a
professor in the Bloomberg School's
Department of
International Health.
From April 2003 to July 2004, the study authors
collected 3,760 sputum — saliva mixed with
respiratory discharge — in order to complete
sensitivity and susceptibility studies of the MODS test.
All samples were decontaminated and then used for parallel
MODS testing, as well as for two current gold standard TB
tests — the Lowenstein-Jensen culture and automated
mycobacterial culture (using the MBBacT system). In order
to test the samples using MODS, the researchers observed
the cultures under an inverted light microscope for 40
days. The sensitivity of detecting TB was 97.8 percent for
MODS culture, 89 percent for automated mycobacterial
culture and 84 percent for Lowenstein-Jensen culture, when
compared to each other. The median time for detection of TB
was seven days for MODS culture, 13 days for automated
mycobacterial culture and 26 days for Lowenstein-Jensen
culture.
The researchers also introduced isoniazid, rifampin,
ethambutol and streptomycin — TB antibiotics —
to the cultures in order to detect drug-resistant TB.
Compared to standard culture reference methods,
drug-resistant susceptibility agreement for MODS was 100
percent for rifampin, 97 percent for isoniazid, 99 percent
for rifampin and isoniazid (combined for multi-drug
resistance), 95 percent for ethambutol and 92 percent for
streptomycin. MODS results were available, on average, in
seven days, making them clinically relevant. Standard
antimicrobial tests can take up to several months.
The MODS test relies on three principles:
Mycobacterium tuberculosis grows faster in liquid medium
than in solid medium; TB grows in characteristic tangles or
coils, and these characteristic cord formations can be seen
microscopically in liquid medium; and the ability to
incorporate drugs permits rapid and direct
drug-susceptibility testing at the same time as the
detection of bacterial growth.
More than 5,000 people die of TB every day worldwide,
and the poor are disproportionately affected, said David
A.J. Moore, lead author of the study and senior lecturer in
infectious diseases and tropical medicine at Imperial
College London. "This is a curable illness. MODS was
developed for developing countries in developing
countries."
The study authors acknowledge that MODS testing
requires a trained technician and a biosecure environment
in which to perform the test. However, they note that the
10-day training period is comparable to testing currently
done to read malaria smears. MODS is a simple, rapid test
that the authors expect will have a major impact in
developing countries where resources are limited.
Co-authors of the study from Johns Hopkins are Robert
H. Gilman, Carlton A. W. Evans and Luz Caviedes. Additional
co-authors are David A.J. Moore, Jorge Coronel, Aldo Vivar,
Eduardo Sanchez, Yvette Pinedo, Juan Carlos Saravia, Cayo
Salazar, Richard Oberhelman, Maria-Graciela Hollm-Delgado,
Doris LaChira, A. Roderick Escombe and Jon S. Friedland.
The study was supported by grants from the Wellcome
Trust and a National Institutes of Health Fogarty Global
Research Training Grant. Co-authors also received grants or
payment for consultant work from Sequilla, Merck, Chiron
and Cerexa.
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2006
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