Assessment of the quantitative ability of AdvanSure TB/NTM real-time PCR in respiratory specimens by comparison with phenotypic methods.

Accurate quantification of mycobacterial load is important to evaluate disease severity and to monitor the course of treatment in tuberculosis (TB). We evaluated the quantitative capability of the AdvanSure TB/NTM real-time PCR kit (LG Life Science, Korea) to determine the cycle threshold (Ct) for mycobacterial burden. We retrospectively analyzed data from 108 patients whose respiratory specimens (sputums and bronchoalveolar lavage fluids) were positive for Mycobacterium tuberculosis complex (85 culture-positive and 23 culture-negative specimens). We compared Ct values with grades of acid-fast bacilli (AFB) staining, semi-quantitative colony count on solid medium, and time to positivity (TTP) in liquid and solid media. We also investigated the cutoff Ct value for predicting stain-positive status. Ct value showed significant reverse correlation with AFB staining grade (r s=-0.635, P<0.01). Ct value significantly decreased as the semi-quantitative counts on the solid medium increased (P<0.001), and the mean Ct value of each of the groups 1+, 2+, 3+, and 4+ were 29.0, 30.0, 27.1, and 25.5, respectively. A weak correlation between Ct value and TTP in liquid and solid media was observed (r s=0.468 and 0.365, respectively). A cutoff Ct value of <33.2 best predicted stain positivity, with a sensitivity of 95.0% and a specificity of 32.0%. Our findings suggest the potential use of AdvanSure TB/NTM real-time PCR kit for quantitatively determining bacterial burden, albeit with some enhancements.