Nucleic acid amplification tests have become popular for the rapid detection of M. tuberculosis in suspected TB patients. However, culture remains the gold standard method for the diagnosis of TB. Egg-based media such as Löwenstein-Jensen are widely used, but agar media such as Selective 7H11 and liquid-based media are now considered standard tools . Several centers also use these systems to isolate rare mycobacterial species. Recently, TK SLC-L (Salubris, Inc.), a liquid version of the biphasic medium TK SLC, was introduced to the market. It enables the detection of M. tuberculosis complex (MTC) in clinical samples based on colourimetry. The culture system evaluated in this study incorporates several innovations, which make this new system convenient and feasible for microbiology laboratories.
The detection of M. tuberculosis in 146 specimens using TK SCL-L and MGIT produced similar results for 110 (75.3%) clinical specimens. A total of 23 (15.8%) cases were positive, and 87 (59.6%) cases were negative by both TK SCL-L and MGIT. However, discordance was obtained for 36 (24,7%) cases. TK SLC-L performed as well as MGIT in the isolation of mycobacteria from sputum samples. Although the difference was not statistically significant, TK SCL-L was superior to MGIT by isolating more mycobacteria among culture-positive specimens. These results are in agreement with the general acceptance of liquid culture systems as being superior to egg-based solid media .
In previous studies, the reported contamination rate for liquid culture systems was 10.0- 20.1%. TK SCL-L had a contamination rate of 1.3%, which is very low compared to that reported for other systems [2, 9]. Although decontamination and concentration of the samples was done by the same technicians, and the same processed samples were inoculated into both TK SCL-L and MGIT, the contamination rate for MGIT (13.1%) was significantly higher than that for TK SCL-L (1.3%). This may be due to the pretreatments required for MGIT tubes. Rehydrating OADC and selective antimicrobials requires extra time and effort, and increases the risk of contamination. TK SLC-L liquid medium, which is used for the inoculation of clinical samples, contains selective antimicrobials to minimise contamination. All types of TK Media are ready-to-use; they do not require preparatory work prior to use. This eliminates the risk of contamination due to the extra manipulations required by other systems.
Sorlozano et al. (2009) compared MGIT, MB/BacT ALERT 3D, and LJ, and reported the time to growth detection as 15.1, 20.2, and 32.4 days, respectively . In another study done by Saitoh and Yamane (2000), MGIT detected mycobacteria in 20 days, on average, compared to 17 days for MB/BacT . In a multicentre study, the average time to growth detection for MGIT, BACTEC 460, and LJ was 13.3, 14.8, and 25.6 days, respectively . Our average time to growth detection for MGIT (13.1 days) was comparable to previously published values. The average and median times to growth detection for TK SCL-L were 18.3 and 15.1 days, respectively. Although the time to growth detection was shorter for MGIT, there was no statistically significant difference between MGIT and TK SCL-L. However, the contamination rate was significantly higher for MGIT. The total time spent for the repetition of cultures for contaminated samples in MGIT make the total return time for culture results equal to or longer than the time required by TK SLC-L.
When the performance of TK SCL-L and MGIT were compared for the diagnosis of MTC, both systems did equally well in recovery and there were no differences in the sensitivity, specificity, positive predictive, negative predictive, and accuracy values. However, the correlation value between total culture positivity and MGIT positivity was less than that for TK SCL-L. This failure to recover mycobacteria by culture in some MGIT tubes was mostly due to the high contamination rate. In other studies, contamination rates between 2.0 and 20.1% have been reported for MGIT [3, 9].