Evaluation of the BACTEC radiometric method for recovery of mycobacteria and drug susceptibility testing of Mycobacterium tuberculosis from acid-fast smear-positive specimens

Abstract

Respiratory specimens (463), all smear positive for acid-fast bacteria, were inoculated onto routine solid media and into BACTEC 7H12 Middlebrook medium for detection of mycobacterial growth. Conventional drug susceptibility testing (1% proportion method) was performed on Middlebrook 7H10/7H11 medium; radiometric susceptibility testing was performed on BACTEC 7H12 medium. The average detection times for BACTEC-positive cultures were 8.3 days for M. tuberculosis and 5.2 days for other mycobacteria; by conventional methods, they were 19.4 and 17.8 days, respectively. These detection times did not include time required for identification, which was done by the conventional method only. There was an excellent correlation in the recovery rates of mycobacteria by the 2 methods. Drug susceptibility test results of M. tuberculosis isolates by the 2 methods showed 95.1-100% overall agreement. The average reporting time for drug susceptibility results ranged from 4.2-6.9 days for the BACTEC method and 13.7-21 days for the conventional methods. An average of 18 days was required for the BACTEC method for complete recovery and drug susceptibility testing of M. tuberculosis, as compared with 38.5 days for the conventional methods.