WHO endorses MODS, NRI and CRI to assess TB in resource-limited settings

In a policy statement by the World Health Organization on July 2010, MODS, CRI, and NRA assays have been given the vote of confidence by the international agency for use by laboratory officials and health care providers for diagnosis of MDR-TB (multi-drug resistant tuberculosis) cases.

Colorimetric redox indicator (CRI) methods comprise growing Mycobacterium tuberculosis isolates in conventional culture. A microscopically observed drug susceptibility (MODS) assay comprises observing micro-colony growth and typical cord-formation of M. tuberculosis in sealed microtitre plates containing liquid culture medium, through an inverted microscope. Nitrate reductase assay (NRA), on the other hand, involves a direct test on smear-positive sputum specimens and an indirect test on M. tuberculosis isolates grown from conventional solid culture.

All three methods exhibit high specificity and sensitivity. These criteria, among others, have led the WHO to recommend the use of selected non-commercial culture and DST methods as an interim solution in resource-constrained settings, under clearly defined programmatic and operational conditions, while capacity for genotypic and/or automated liquid culture and DST are still being developed.



Reference: World Health Organization. 2010, July. Non-Commercial Culture And Drug-Susceptibility Testing Methods For Screening Of Patients At Risk Of Multi-Drug Resistant Tuberculosis: Policy Statement. Retrieved November 15, 2010. Accessed at http://www.who.int/tb/dots/laboratory/whopolicy_noncommercialculture_and_dstmethods_july10.pdf.