How to compare two variables within the same cohort of patients to prove or disprove that there is a significant statistical difference between them?

I am doing an observational cohort study involving measurement of procalcitonin PCT on a daily basis in a cohort of patients with infection who are on antibiotics. When PCT concentration reaches a level of <0.25 ng/ml it can be taken that on this day that infection has been erradicated and antibiotics could have been potentially stopped. I want to compare the potential stop day against the actual stop day on antibiotics to determine if PCT can be used to reduce duration of antibiotics.
Therefore what statistical analysis do i use? Is it an independent t-test comparing potential stop against actual stop in a patient receiving antibiotics?