Method for separating serum or plasma from the formed elements of blood

A method of separating blood into a light phase comprising plasma or serum and a heavy phase comprising the formed elements of the blood including a separator assembly comprising a container having one end and another end, said container being adapted to receive blood from subsequent separation into the light phase and the heavy phase, piston means having an average specific gravity greater than said light phase, the piston means including a container sealing means adapted to sealingly engage the container wall at the periphery of the container, a valve seating portion having a valve opening therein, said valve opening permitting serum or plasma to flow through the piston means, a movable valve member adapted to seat on the seating portion so as to close the valve opening, and bias means connected between the valve member and the valve seating portion, said method comprising the following steps:

• centrifuging the formed elements of the blood toward one end of the container so as to form the heavy phase and the plasma or serum toward the other end of the container so as to form the light phase;
• centrifuging the piston means through the container toward the interface of the centrifuged formed elements and the centrifuged plasma or serum;
• overcoming the seating force maintained by the bias means on the valve member during centrifuging so as to open the valve opening;
• and
• maintaining a substantial seating force on the valve member through the bias means when the piston means comes to rest in the container so as to close the valve opening;
• said force being sufficiently great to maintain said valve member in a seated position when said valve member comes to rest in the container at the interface between said light phase and said heavy phase.