Evaluation of a modified direct agar proportion method for testing susceptibility of Mycobacterium tuberculosis from MGIT samples

Background/Purpose: The emergence of resistance to anti-tuberculosis (TB) drugs has become an obstacle to effective TB control. Thus, there is an urgent need to identify patients and initiate adequate treatment for drug-resistant cases in a timely manner. The BACTEC MGIT 960 system is well known for its rapid culturing time, and is in widespread use in Taiwan. In this study, we evaluated the possibility of replacing the traditional indirect agar proportion method with a modified direct agar proportion method (MDAPM), as a technique for rapid testing the drug susceptibility of Mycobacterium tuberculosis without additional cost. Methods: In this study, 432 positive MGIT 960 samples that were identified as M. tuberculosis complex using the MeDiPro M. tuberculosis Antigen Rapid Test or the Cobas Amplicor MTB test were evaluated. Each sample was tested separately by the MDAPM and indirect agar proportion method, between July 2008 and December 2008, to compare the consistency and total turnaround time. Results: Four first-line anti-TB drugs-rifampin, isoniazid, ethambutol, and streptomycin-were tested. For the MDAPM and indirect agar proportion method, the respective consistencies for each drug were 99.31%, 98.38%, 98.38%, and 97.22%. Our results also indicated that the MDAPM leads to an average saving in working time of 2 weeks, compared with the traditional indirect agar proportion method. Conclusion: In addition to having the potential to shorten turnaround time without compromising diagnostic quality, the MDAPM also provides a more efficient and cost-effective procedure. This modified procedure presents potential benefits for TB diagnosis in laboratories already equipped with the MGIT 960 system.