Time to culture positivity (TTP) in liquid medium is now widely available as a measure of viable mycobacterial sputum load. TTP correlates well with and could replace colony-forming unit (CFU) counting in studies of antituberculosis drug effects. We investigated the influence of the pre-treatment mycobacterial sputum load on 4428 CFU measurements obtained within the first 14 days of treatment. Using a prediction model we show that pre-treatment CFU counts contribute 29% to the variation of on-treatment CFU counts and increase the precision of the prediction of on-treatment CFU from TTP by 12%. On the other hand, pre-treatment TTP contributed only 12% to the variation of on-treatment TTP and only added 2% to the prediction of TTP from CFU. We conclude pre-treatment measurements are covariates that can enhance the accuracy of statistical estimates of treatment effects, particularly when measured by CFU counts.