Rapid prediction of BACTEC MGIT 960 culture results by COBAS amplicor mycobacterium polymerase chain reaction detection

Abstract

A combination of a rapid culture system, BACTEC MGIT 960 (MGIT), and a commercial polymerase chain reaction (PCR) system for the detection of Mycobacterium tuberculosis was evaluated to predict final culture results within 2 weeks. A total of 79 sputum specimens were collected from 59 tuberculosis (TB) patients before anti-TB chemotherapy. Among the 22 specimens that were smear negative and culture positive, the COBAS Amplicor nucleic amplification method for sputum resulted in 13 positives (59.1%) before culturing. In contrast, 21 liquid culture specimens (95.5%) showed positive results by COBAS Amplicor after 7 days. Similarly, 8 specimens (80%) were positive for Mycobacterium avium complex (MAC) based on COBAS Amplicor, and 10 liquid culture specimens (100%) showed positive results after 7 days. Among the 26 specimens that took more than 7 days to become positive by MGIT, 25 specimens (96.1%) were positive using COBAS Amplicor with 7-day-old cultures. Of the 26 positives, 21 were M. tuberculosis, which took 11 to 38 days to appear positive (mean, 16.6 days), and 4 were MAC, which took 8 to 10 days (mean, 8.8 days). As a result, 96.8% (31/32) of the positives could be detected by MGIT with COBAS Amplicor by day 7, and the negative predictive value was 97.9%. A combination of MGIT and COBAS Amplicor on day 7 was demonstrated as a useful method for rapid diagnosis of positives and negatives, without waiting 42 days for confirmation.