Multicenter evaluation of the mycobacteria growth indicator tube for testing susceptibility of Mycobacterium tuberculosis to first-line drugs.

In a multicenter study involving three reference centers for mycobacteria, the reliability of the Mycobacteria Growth Indicator Tube (MGIT) for rapid antimicrobial susceptibility testing (AST) of Mycobacterium tuberculosis was evaluated and compared to the radiometric method (BACTEC 460TB). Test cultures for which the results of the MGIT and BACTEC 460TB tests were discordant were checked by the conventional proportion method on solid medium. Four hundred forty-one isolates have been tested for susceptibility to isoniazid (INH), rifampin (RMP), ethambutol (EMB), and streptomycin (SM). Discrepant results were obtained for three isolates (0.7%) with INH (susceptible by MGIT, resistant by BACTEC 460TB), for four isolates (0.9%) with RMP (susceptible by MGIT, resistant by BACTEC 460TB), for six isolates (1.9%) with EMB (four susceptible by MGIT, resistant by BACTEC 460TB; two resistant by MGIT, susceptible by BACTEC 460TB), and for four isolates (0.9%) with SM (two susceptible by MGIT, resistant by BACTEC 460TB; two resistant by MGIT, susceptible by BACTEC 460TB). When cultures with discordant results were tested by the conventional proportion method, about half of the cultures yielded results similar to the BACTEC 460TB results, while the other half yielded results similar to the MGIT results. Turnaround times were 3 to 14 days (median, 8.8 days) for MGIT and 3 to 15 days (median, 7.8 days) for BACTEC 460TB. There was no statistically significant difference between the susceptibility testing results of the two methods (P > 0.05). These data demonstrate that the MGIT system is an accurate, nonradiometric alternative to the BACTEC 460TB method for rapid susceptibility testing of M. tuberculosis.

Rapid detection of mycobacteria in clinical specimens by using the automated BACTEC 9000 MB system and comparison with radiometric and solid-culture systems.

Recovery rates of acid-fast bacilli (AFB) and the mean time to their detection from clinical specimens were determined by using the fluorescent BACTEC 9000 MB system. Data were compared to those assessed by the radiometric BACTEC 460 system and by cultivation on solid media. A total of 3,095 specimens were processed with N-acetyl-L-cysteine-NaOH by two laboratories. The contamination rates for the BACTEC 9000 MB system were 6.8% (center 1) and 9.8% (center 2). A total of 451 mycobacterial isolates were detected (Mycobacterium tuberculosis complex, n = 296; nontuberculous mycobacteria [NTM], n = 155). These isolates originated from 94 (20.8%) smear-positive and 357 (79.2%) smear-negative specimens. The BACTEC 9000 MB system was significantly better than solid media (P < 0.05) in detecting AFB, but it was less efficient than the radiometric system (P < 0.01). The BACTEC 9000 MB system plus solid media (combination A) recovered 393 (87.1%) of the isolates, while the BACTEC 460 system plus solid media (combination B) detected 430 (95.3%) of all AFB isolates. Between combination A and B there was no statistically significant difference for the detection of isolates from smear-positive specimens (P > 0.05), in contrast to the recovery of AFB from smear-negative specimens for M. tuberculosis complex, P < 0.05; for NTM, P < 0.01). The mean time to detection of M. tuberculosis complex was 12.2 days for smear-positive specimens and 18.1 days for smear-negative specimens with the BACTEC 9000 MB system; 9.3 and 15.6 days, respectively, with the BACTEC 460 system; and 21.2 and 28.4 days, respectively, with solid media. For NTM, the average detection times were 15.1, 17.3, and 31.3 days by the three methods, respectively. In conclusion, the BACTEC 9000 MB system is a rapid, less labor-intensive detection system which allows for higher levels of recovery of AFB than solid media. There is no risk of cross contamination, which is known to be the case for the BACTEC 460 system, and data management is greatly facilitated. As a whole, however, the BACTEC 9000 MB system should only be used in conjunction with solid media.

Pretreatment of clinical specimens with sodium dodecyl (lauryl) sulfate is not suitable for the mycobacteria growth indicator tube cultivation method.

When using the Mycobacteria Growth Indicator Tube (MGIT), pretreatment of clinical specimens with N-acetyl-L-cysteine-NaOH is recommended by the manufacturer. Processing of clinical specimens (n = 1,000) with sodium dodecyl (lauryl) sulfate-NaOH resulted in both poor recovery and delayed mean time to detection of acid-fast bacilli. Values were comparable to those obtained on solid media.

Comparison of the Mycobacteria Growth Indicator Tube (MGIT) with radiometric and solid culture for recovery of acid-fast bacilli.

In a multicenter study involving three reference centers for mycobacteria, the rate of recovery of acid-fast bacilli (AFB) and the mean time to their detection from clinical specimens was determined by using the Mycobacteria Growth Indicator Tube (MGIT). These parameters were compared to those assessed by the radiometric BACTEC 460 TB system and by cultivation on solid media. Clinical specimens (n = 1,500) were pretreated with N-acetyl-L-cysteine (NALC)-NaOH. The contamination rates for MGITs were 2.0% (center 1), 13.8% (center 2), and 6.1% (center 3). A total of 180 mycobacterial isolates were detected (M. tuberculosis complex, n = 113; nontuberculous mycobacteria [NTM], n = 67). When using a combination of liquid and solid media (the current "gold standard" for culture), MGIT plus solid media detected 156 (86.7%) of the isolates, whereas BACTEC plus solid media recovered 168 (93.3%) of all AFB. Between these two gold standards there was no statistically significant difference (P > 0.05). The combination of MGIT plus BACTEC detected 171 (95.0%) of all isolates (compared with MGIT plus solid media, P < 0.01; compared with BACTEC plus solid media, P > 0.05). Considering the efficacies of the different media separately, MGIT was superior to solid media (although not significantly; P > 0.05) in detecting AFB but was inferior to the BACTEC system (P < 0.01). The mean time to the detection of M. tuberculosis complex was 9.9 days with MGIT, 9.7 days with BACTEC, and 20.2 days with solid media. NTM needed, on average, 11.9, 13.0, and 22.2 days to appear by the three methods, respectively. In conclusion, MGIT proved to be a valuable alternative to the radiometric cultivation system.

Diagnostic performance of amplified Mycobacterium tuberculosis direct test with cerebrospinal fluid, other nonrespiratory, and respiratory specimens.

The Gen-Probe Amplified Mycobacterium tuberculosis Direct Test (MTD) was adapted to be used for cerebrospinal fluid (CSF) and a large variety of other nonrespiratory specimens. Standardized with artificially spiked dilution series of CSF, the modified MTD procedure consists of (i) increasing the amount of sample 10-fold, (ii) pretreating the specimen with a detergent, and (iii) increasing the amplification time from 2 to 3 h. Performance of MTD in a clinical mycobacteriology laboratory was tested over an extended period of time, involving a total of 322 nonrespiratory as well as 1,117 respiratory specimens from 998 patients. Results from MTD were compared with those from microscopy, culture, analysis of tuberculostearic acid by gas-liquid chromatography-mass spectrometry (CSF only), and the final clinical diagnosis. When MTD results were compared with resolved data, the sensitivity, specificity, and positive and negative predictive values for MTD were 93.1, 97.7, 90.0, and 98.5%, respectively, for nonrespiratory specimens and 86.6, 96.4, 76.8, and 98.1%, respectively, for respiratory specimens. Our data demonstrate that (i) MTD is a robust, highly sensitive and specific technique for the rapid detection of M. tuberculosis complex in all types of clinical specimens, (ii) there was no statistically significant difference (P > 0.005) in sensitivity and specificity for nonrespiratory compared with respiratory specimens, and (iii) repeating all MTDs which yield a result between 30,000 and 200,000 relative light units would help prevent a large number of false positives and, thus, enhance test specificity.