Objectives: To evaluate nitrate reductase assay (NRA) efficacy for streptomycin, isoniazid, rifampicin and ethambutol susceptibility testing of Mycobacterium tuberculosis strains.
Methods: Results were generated by three laboratories: the Instituto Adolfo Lutz (IAL) Mycobacteria Reference Laboratory and two IAL Regional Laboratories in Santo André and Sorocaba, São Paulo State, Brazil. One hundred and twenty M. tuberculosis strains were simultaneously tested using NRA and the proportion method (PM), while 117 strains were tested using both NRA and BACTEC MGIT 960 (M960).
Results: Repeatability analysis of NRA results showed rates of 100% for isoniazid and ethambutol and 97% for streptomycin and rifampicin susceptibility detection, representing substantial agreement. McNemar testing of the data also indicates that NRA and PM, as well as NRA and M960, do not differ significantly. On average, NRA results were available after 10 days.
Conclusions: The data demonstrate that NRA is reliable for susceptibility testing of isoniazid and rifampicin, the two most important drugs for the treatment of tuberculosis. In addition, the reduction in the time necessary to obtain susceptibility results is of fundamental importance.