A simple, low-cost and low-tech microscopic observation drug susceptibility (MODS) TB culture assay for detection of drug-resistant TB for use in resource-limited settings
S. Suniti1, P. Balakrishnan1, R. Vignesh1, S.S. Sunil1, K.G. Murugavel1, D. Moore2
1YRG Centre for AIDS Research and Education (YRG CARE), Chennai, India, 2AB PRISMA and Universidad Peruana Cayetano Heredia, Lima, Peru
Background: TB is
a major public health problem in the developing world and continues to be a
leading cause of morbidity and mortality.
Global efforts for TB control are being challenged by the steady
increase in drug-resistant TB. The objective of the present study is to
evaluate the performance of MODS assay to detect drug-resistant TB in
comparison to BD-BACTEC TB460 system.
Methods: The MODS is a simple
24-well plate based assay utilizing observation of Middlebrook 7H9 cultures
with an inverted light microscope to detect the characteristic tangles or cords
of M.tuberculosis in liquid media and
incorporation of anti-TB drugs enables direct susceptibility testing with
sputum. The performance of MODS in detecting drug-resistant (RIF
and INH) strains was compared with that of BACTEC. A total of 315 sputum specimens from patients
where TB was suspected were processed by both BACTEC and MODS. 164 specimens
were from HIV patients and 151 from patients, whose HIV status was not known.
Results: The results of BACTEC
susceptibility test were available for 27 strains and were compared with MODS;
of which, all 21 strains were susceptible to both the drugs by both the assays
and all 6 resistant strains (two multi-drug resistant (MDR), three
mono-resistant to INH and one mono-resistant to RIF)
detected by BACTEC were also concordantly detected by MODS. Interestingly, MODS
detected an additional MDR strain, which was missed out by BACTEC. The median duration for detecting
resistant strains by MODS and BACTEC were 9 days (range 3 to 18) and 22.5 days
(range 14 to 36), respectively.
Conclusions: The MODS assay appears to be
sensitive and specific. Therefore, MODS could be an alternative method for detecting
MDR in resource-limited settings, where the prevalence of TB is high and access
to time-consuming conventional and expensive automated TB culture systems are very
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