A filter vial assembly, comprising: a cylindrical walled vial with a closed bottom and open top with walls defining a cylindrical interior having a diameter D, the vial having a hollow, tubular piston therein, the piston having a distal end covered by a cup having a proximal cup seal extending outward from the proximal end of the cup a distance greater than the diameter D, the cup seal being sized relative to the inner walls of the vial to form a fluid tight seal when the piston is advanced into the vial a distance sufficient to engage the proximal cup seal with the vial walls, the distal end of the piston having a piston flange extending outwardly and located adjacent an exterior facing recess in the piston so the cup forms a snap fit over the piston flange and extends into the recess, the piston flange causing the cup to bulge sufficiently outward to form a distal fluid tight cup seal with the vial when the flange and cup are inserted into the vial; and a filter having a periphery abutting a lip on the cup that extends inward toward the longitudinal axis and held between a piston support at the distal end of the piston and the lip; wherein the cup has a sidewall extending between a proximal and distal ends of the cup with interior diameter smaller than the distal piston flange, and the sidewall of the cup is sufficiently flexible to deform outward as the cup is passed over the piston flange so the sidewalls assume an outer dimension larger than diameter D at the location of the piston flange when the retainer cup is passed over the piston flange; wherein fluid or filtrate is forced through the filter and into the hollow interior of the tubular piston when the assembled cup and piston is fitted into the vial; characterized in that: as the cup is fitted over the recessed portion and the piston flange, the sidewall of the cup is deformed outward so that the outer diameter of the assembled cup and piston is larger than the diameter of the piston body, and wherein the piston flange bulges the sidewall of the cup outward enough to form a fluid tight seal with the inner walls of the vial; and wherein the piston, cup, and vial are integrally molded. wherein the cup has a sidewall extending between a proximal and distal ends of the cup with interior diameter smaller than the distal piston flange, and the sidewall of the cup is sufficiently flexible to deform outward as the cup is passed over the piston flange so the sidewalls assume an outer dimension larger than diameter D at the location of the piston flange when the retainer cup is passed over the piston flange; wherein fluid or filtrate is forced through the filter and into the hollow interior of the tubular piston when the assembled cup and piston is fitted into the vial; characterized in that: as the cup is fitted over the recessed portion and the piston flange, the sidewall of the cup is deformed outward so that the outer diameter of the assembled cup and piston is larger than the diameter of the piston body, and wherein the piston flange bulges the sidewall of the cup outward enough to form a fluid tight seal with the inner walls of the vial; and wherein the piston, cup, and vial are integrally molded. as the cup is fitted over the recessed portion and the piston flange, the sidewall of the cup is deformed outward so that the outer diameter of the assembled cup and piston is larger than the diameter of the piston body, and wherein the piston flange bulges the sidewall of the cup outward enough to form a fluid tight seal with the inner walls of the vial; and wherein the piston, cup, and vial are integrally molded.