COBAS® TaqMan® MTB, smear positivity grade and MGIT culture; correlation analyses of three methods for bacillary quantification

We investigated the correlation between the cycle threshold (Ct) value of the COBAS® TaqMan® MTB (TaqMan MTB), the mycobacterial smear positivity grade, and the time to detection (TTD) in the Mycobacteria Growth Indicator Tube (MGIT) for quantification of Mycobacterium tuberculosis (MTB). For 57 sputum samples, significant correlations were observed between the Ct value, the smear positivity grade, and the MGIT TTD (Spearman's rank correlation coefficient: rs = −0.940, P < 0.001 and Pearson's correlation coefficient: rp = 0.737, P < 0.001). In addition, a correlation was observed between the number of bacteria estimated based on the smear positivity grade and the number of MTB bacilli calculated by the Ct value (rs = 0.930, P < 0.001). This study has demonstrated the possible estimation of the smear positivity grade and MGIT TTD using the Ct value of TaqMan MTB, which is based on a real-time PCR system, for diagnostic samples.