Evaluation of rifampicin and isoniazid susceptibility testing of Mycobacterium tuberculosis by a mycobacteriophage D29-based assay.

Conventional methods for determining drug susceptibility of Mycobacterium tuberculosis require several weeks to obtain results, limiting their usefulness; automated methods and those based on molecular biology techniques have been able to reduce the turnaround time, but their high cost and need for sophisticated equipment restrict their use in developing countries. The goal of the present study was to evaluate the diagnostic accuracy of a rapid (3-4 days) low-cost test based on the use of mycobacteriophage D29 to determine the susceptibility of strains of M. tuberculosis to rifampicin (RIF) and isoniazid (INH). Results obtained show that susceptibility testing for RIF has a high diagnostic accuracy as compared to the standard indirect proportion method on Löwenstein-Jensen medium (sensitivity 100% and specificity 98%). Given the association between the resistance to RIF and INH, which define multidrug resistance (MDR), this test seems suitable for rapid detection of MDR tuberculosis strains (kappa=0.978). Susceptibility testing for INH using mycobacteriophage D29 had a good but lower diagnostic accuracy as compared to the standard method (sensitivity 80.4% and specificity 80.8%); the test would then be of limited usefulness in the management of tuberculosis patients. Further studies to determine the relationship of mycobacteriophage D29 tests to in vivo correlates of sensitivity to specific antituberculosis drugs are warranted.