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However, we would like to stress that there are other options that have recently been described2 and are currently under evaluation. As compared with MODS culture, the nitrate reduction assay, based on a simple procedure involving the use of Löwenstein–Jensen medium, has been tested in sputum samples with similarly good results.3 The thin-layer agar method, which is similar to MODS culture but with solid medium and standard microscopes, had better results than conventional methods when evaluated in target populations.4 In ongoing evaluations, the thin-layer agar method has also outperformed the reference method for detecting multidrug-resistant tuberculosis. In addition, direct colorimetric methods with redox indicators have performed very well5 and are under further evaluation. A disadvantage of the MODS method remains the requirement of an inverted microscope, which is not routinely available in laboratories that perform diagnostic tests for tuberculosis.
Juan-Carlos Palomino, Ph.D.
Anandi Martin, Ph.D.
Francoise Portaels, Ph.D.
Institute of Tropical Medicine
2000 Antwerp, Belgium
jcpalomino{at}itg.be
References
Rumina Hasan, M.B., B.S., Ph.D.
Seema Irfan, M.B., B.S.
Aga Khan University
Karachi 74800, Pakistan
rumina.hasan{at}aku.edu
References
In our opinion, MODS culture is actually safer than any indirect drug-susceptibility testing method, since culture amplification and direct drug-susceptibility testing occur within a closed system: the MODS plate is inoculated and then sealed within a ziplock bag. It is not manipulated again, since all readings, including those for drug-susceptibility testing, are done through the bag. The handling of cultured M. tuberculosis at bacterial concentrations thousands of times those of clinical specimens, which is required for secondary drug-susceptibility testing, dwarfs the biohazard risk associated with sputum-decontamination processes common to all culture methods. This handling risk is entirely avoided with the MODS assay.
Hasan and Irfan did not use ziplock bags, because this important detail was omitted from previous articles on the MODS assay.1,2,3 We do not believe that a MODS laboratory needs to meet biosafety level 3 standards. Combining the use by laboratory staff of respirators approved by the National Institute for Occupational Safety and Health and appropriate protective clothing with a well-positioned, properly maintained class II biologic safety cabinet that recirculates exhausted air through a high-efficiency particulate air (HEPA) filter into a closed room, should be adequate. With respect to rapidly growing mycobacteria, these organisms should overgrow MODS plates by day 5, a phenomenon not seen with M. tuberculosis.
David A.J. Moore, M.D.
Imperial College Wellcome Centre for Clinical Tropical Medicine
London W12 0NN, United Kingdom
davidajmoore{at}msn.com
Robert H. Gilman, M.D.
Johns Hopkins Bloomberg School of Public Health
Baltimore, MD 21205
Jon S. Friedland, M.D., Ph.D.
Imperial College Wellcome Centre for Clinical Tropical Medicine
London W12 0NN, United Kingdom
References
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