Noncommercial culture and drug-susceptibility testing methods for screening patients at risk for multidrug-resistant tuberculosis
Several noncommercial culture and drug-susceptibility testing (DST) methods have been developed specifically for settings with limited access to sophisticated laboratory infrastructure and technical expertise. Several rapid, inexpensive methods have shown initial promise. The most advanced are microscopic observation of drug susceptibility (MODS), colorimetric redox indicator (CRI) methods, thin-layer agar methods, the nitrate reductase assay (NRA) and mycobacteriophage-based assays.
In 2009, the strength of the evidence for these noncommercial methods was assessed by WHO, following standards appropriate for evaluating both the accuracy of new TB diagnostics and their effect on patients and public health. The results showed that the current evidence is insufficient to recommend the use of thin-layer agar or phage-based assays. There was considered to be sufficient evidence for the use of CRI methods, MODS and NRA under clearly defined programme and operational conditions, in reference laboratories and under strict laboratory protocols, and as an interim solution while capacity for genotypic or automated liquid culture and DST is being developed.
Under these conditions, MODS and NRA are recommended for direct testing of sputum specimens. Together with CRI methods, MODS and NRA are also recommended for indirect DST of M. tuberculosis isolates grown in conventional culture. The time to detection of MDR-TB may not necessarily be faster with indirect testing, and none of these methods can detect extensively drug-resistant TB. Conventional culture and DST capacity are therefore still required in all settings.