Serum/plasma separator-strut stop type

A blood collection and separator assembly of the type suitable for use in centrifuging blood to separate the plasma or serum, the light phase, from the cellular portion, the heavy phase. The assembly includes a collection container and a piston disposed therein for sealing off one phase from the other after centrifuging is terminated. The piston is formed having an average specific gravity heavier than the light phase of the blood. The piston is slidably disposed in the container with its outer surfaces in sealing contact with the inner surface of the container and is provided with means which, under centrifugal force, permit the light phase of the blood to pass the piston as the piston moves down through the light phase while retaining sealing engagement with the inner surface of the container. The piston is provided with means to stop the piston at a predetermined distance above the bottom of the container whereupon the piston can serve as an impervious barrier between the two phases of the blood.