Microscopic Observation Drug-Susceptibility Assay
The Microscopic Observation Drug Susceptibility Assay (MODS) is a liquid culture method based on microscopic detection of characteristic M. tuberculosis morphology.21 MODS uses two well-known properties of Mycobacterium tuberculosis.
- The rate of growth in liquid medium considerably quicker than that on solid medium
- The morphology of M. tuberculosis in liquid culture is characteristic and recognizable, consisting of tangles or cords of organisms
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By use of an inverted light microscope to examine 24-well plates inoculated with Middlebrook /H0-broth and decontaminated sputum, mycobacterial growth can thus be detected long before it would be visible to the naked eye. Incorporation of anti-
TB drugs at the
outset enables direct susceptibility testing with clinical specimens.
MODS is a low technology, direct DST method which detects resistance to isoniazid and rifampicin directly from sputa, with positive results available within 2 weeks in most cases.23 MODS had a shorter time to culture positivity (average of 8 days) compared with LJ culture. Overall MODS appears to be a promising, novel, inexpensive tool that can rapidly detect TB and drug resistance directly from sputum specimens.
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Conclusion
Despite the enormous global burden of tuberculosis, case detection rates continue to be low, jeopardizing global TB control efforts, particularly in areas with high HIV prevalence. Presently TB diagnosis relies on old conventional tools, which has several limitations. The long felt need for new tools is currently being addressed and in the last years, several new diagnostic approaches have been proposed for latent and active tuberculosis detection. New genotypic and phenotypic methods for rapid diagnosis of drug resistance have been developed and tested both in M. tuberculosis strains as well as in clinical samples.
The most important consideration to take into account for new diagnostic methods is that they should be as good as or better than the currently existing tools and, at the same time, be adequate for low-resource countries where the burden of TB is more important. The cost is another important consideration, since at the current prices these commercial kits are still out of the reach of most TB diagnostic laboratories in low-resource countries.
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The main challenge is to ensure that new tools undergo rigorous evaluations in field conditions, and also to make sure that benefits of promising new tools actually reach the populations in developing countries that need them most. Further controlled evaluations are necessary in high-endemic countries for their eventual implementation in the routine diagnostic systems. As long as these constraints are not properly addressed, expensive commercial diagnostic kits will remain restricted to developed countries or academic and research laboratories with the appropriate funding, but far away from of the TB control programmes.