High variance in quantification of Mycobacterium tuberculosis at low bacterial loads and with differentially detectable mycobacteria.

We examined the correlation between three different methods of Mycobacterium tuberculosis quantification: time to positivity (TTP), log 10 CFU, and an assay to detect differentially detectable M. tuberculosis (DD Mtb) from three different prospective studies. Participants with DD Mtb have significantly more variation in the CFU/TTP correlation than participants with no DD Mtb ( P < 0.001). This may impact the design of early bactericidal activity studies that use TTP as the primary outcome.