Provided are methods and systems for the pathogen inactivation of blood products and compositions, including methods of using devices (e.g., comprising component(s) configured to control flow of fluid(s)) to determine the amount of pathogen inactivation compound (PIC) necessary for pathogen inactivation of a blood composition (e.g., based at least in part on one or more input parameters) and allow transfer of the blood composition and the PIC into a container, e.g., of a processing set.