A device and method for separating serum or plasma from blood cells in a whole blood specimen. The present invention uses a cap with a reservoir, such that blood cells are packed into the cap when the specimen container is centrifuged. When the cap is removed, the blood cells are also removed, and the serum or plasma is left in the specimen tube where it can be readily extracted by a pipette which is able to reach all the way to the bottom of the specimen tube minimizing the dead volume.
