Direct Drug Susceptibility Testing of Mycobacterium tuberculosis for Rapid Detection of Multidrug Resistance Using the Bactec MGIT 960 System: a Multicenter Study

Conventional indirect drug susceptibility testing of Mycobacterium tuberculosis with liquid medium is well established and offers time-saving and reliable results. This multicenter study was carried out to evaluate if drug susceptibility testing (DST) can be successfully carried out directly from processed smear-positive specimens (direct DST) and if this approach could offer substantial time savings. Sputum specimens were digested, decontaminated, and concentrated by the laboratory routine procedure and were inoculated in Bactec MGIT 960 as well as Lowenstein-Jensen (LJ) medium for primary isolation. All the processed specimens which were acid-fast bacterium (AFB) smear positive were used for setting up direct DST for isoniazid (INH) and rifampin (RIF). After the antimicrobial mixture of polymyxin B, amphotericin B, nalidixic acid, trimethoprim, and azlocillin (PANTA) was added, the tubes were entered in the MGIT 960 instrument using the 21-day protocol (Bactec 960 pyrazinamide [PZA] protocol). Results obtained by direct DST were compared with those obtained by indirect DST to establish accuracy and time savings by this approach. Of a total of 360 AFB smear-positive sputum specimens set up for direct DST at four sites in three different countries, 307 (85%) specimens yielded reportable results. Average reporting time for direct DST was 11 days (range, 10 to 12 days). The average time savings by direct DST compared to indirect DST, which included time to isolate a culture and perform DST, was 8 days (range, 6 to 9 days). When results of direct DST were compared with those of indirect DST, there was 95.1% concordance with INH and 96.1% with rifampin. These findings indicate that direct DST with the Bactec MGIT 960 system offers further time savings and is a quick method to reliably detect multidrug resistance (MDR) cases.