Disclosed is a tubular piston for a filter vial assembly, wherein the filter vial assembly includes a tubular vial with an open proximal end and a closed distal end and a cylindrical sidewall of diameter D. The piston includes a longitudinal axis and opposing proximal and distal ends and a cup covering the distal end of the tubular piston, the cup having a proximal cup seal engages the walls of the vial to form a fluid tight seal when the piston is advanced into the vial. The piston flange extends away from the longitudinal axis at the distal end the piston and located adjacent a recess in the tubular piston so the cup forms a snap fit over the piston flange and extends into the recess, the piston flange causes the cup to bulge sufficiently outward to form a fluid tight cup seal with the vial when the flange and cup are inserted into the vial. The tubular piston and cup each further include a porous bottom through which fluid can flow during use. The cup includes a continuous sidewall overlapping the distal end of the tubular piston. The sidewall is flexible to deform around the piston flange to achieve a diameter greater than D in a plane orthogonal to the longitudinal axis and through the piston flange but small enough in diameter to fit in the open end of the filter vial. The cup also has a lip and flange that each extend around the cup.
