Abstract: A method and apparatus for determining, controlling or establishing the level of a constituent such as anticoagulant in a body fluid such as blood in a procedure for processing the fluid. The anticoagulant flow rate from an anticoagulant reservoir, the blood flow rate, the flow rate of collected fluid and the flow rate of replacement fluid are monitored, and an anticoagulant flow balance is established whereby the flow of anticoagulant into the donor or patient will be equal to the flow rate of anticoagulant out of the anticoagulant reservoir, plus the flow rate of anticoagulant from the donor or patient, plus the flow rate of anticoagulant in the replacement fluid, minus the flow rate of anticoagulant in the collect fluid. The flow rate of anticoagulant from the donor or patient can be determined based on the anticoagulant metabolic half-life, the volume of the donor's or patient's blood, and the amount and time of infusion of anticoagulant into the donor or patient.