A biological fluids concentration device, including a tube-in-tube assembly, is disclosed. The tube-in-tube assembly receives biologic fluids and may then be placed in the bucket of a centrifuge and spun to separate out the components of the biological fluid by their various densities. For example, whole blood may be centrifuged in the tube-in-tube assembly for separating into plasma, red blood cell component, and a buffy coat. A piston slideably and sealingly engages an inner tube of the tube-in-tube assembly, the inner tube fitting within an outer tube. A lid is designed to engage the top of the outer tube, which lid has an opening therein for receipt of a plunger. The plunger is adapted to move up and down with respect to the lid and the tubes, so as to sealingly, in a down position, and unsealingly, in an open position, engage the top of the inner tube of the tube-in-tube assembly.
