A method and an apparatus are provided for preventing retrograde flow of fluid, e.g., blood products, into a source of sterile substitution fluid (50). The apparatus of the present invention includes a controllable pinch valve member (110) that is placed on a section of a conduit (90) which carries sterile substitution fluid to an extracorporeal circuit (30). In one embodiment, control over the valve member (110) is based on a control unit (120) using fluid pressures that are sensed upstream and downstream of the valve member (110) by upstream sensor (121) and downstream pressure (122) respectively. The valve member (110) is preferably opened only when the upstream pressure is greater than the downstream pressure. This assures that the substitution fluid flows only in a single direction when the pinch valve member (110) completely occludes the conduit (90) when in a closed position. Therefor, blood will not contaminate the sterile fluid by being drawn into the conduit (90) due to pressure differences.