A method of preventing reflux within a syringe barrel of a pre-filled syringe filled with a saline solution and configured for being used to flush catheters, said method comprising the steps of: (a) providing a stopper comprising a main body defining an open rearward end and a closed front end, said open rearward end adapted to receive a front attachment member of a plunger rod therein, a core member integrally formed with said main body adjacent said closed front end, said core member including a nose portion, said stopper characterized by at least one rib extending radially outward around a perimeter of said main body and wherein said main body includes an inner surface adapted for contact with a taper on the front attachment member of the plunger rod, said inner surface of said main body and said taper cooperating together such that the stopper applies a radial force to the syringe barrel upon the application of a forward force to the plunger rod and wherein said core member is interconnected with said main body via a flexible membrane enabling the core member to move independently from the main body; (b) inserting the front attachment member of the plunger rod within the open rearward end of said stopper; (c) applying a force to said plunger rod to advance the stopper into the syringe barrel until the nose portion of the core member contacts an outlet opening at the front end of the syringe barrel forming a seal and trapping fluid from flowing out into the outlet opening; (d) applying additional force to said plunger rod to compress at least a portion of said stopper, so as to advance the at least one rib within the syringe barrel and to compress any trapped air to form increased pressure; and (e) releasing the force on said plunger rod to release the seal between the nose portion and the outlet opening of the syringe barrel wherein friction force maintains said at least one rib in an advanced position relative to a starting position within the syringe barrel such that the increased pressure causes any trapped fluid to be pushed through said outlet opening. (a) providing a stopper comprising a main body defining an open rearward end and a closed front end, said open rearward end adapted to receive a front attachment member of a plunger rod therein, a core member integrally formed with said main body adjacent said closed front end, said core member including a nose portion, said stopper characterized by at least one rib extending radially outward around a perimeter of said main body and wherein said main body includes an inner surface adapted for contact with a taper on the front attachment member of the plunger rod, said inner surface of said main body and said taper cooperating together such that the stopper applies a radial force to the syringe barrel upon the application of a forward force to the plunger rod and wherein said core member is interconnected with said main body via a flexible membrane enabling the core member to move independently from the main body; (b) inserting the front attachment member of the plunger rod within the open rearward end of said stopper; (c) applying a force to said plunger rod to advance the stopper into the syringe barrel until the nose portion of the core member contacts an outlet opening at the front end of the syringe barrel forming a seal and trapping fluid from flowing out into the outlet opening; (d) applying additional force to said plunger rod to compress at least a portion of said stopper, so as to advance the at least one rib within the syringe barrel and to compress any trapped air to form increased pressure; and (e) releasing the force on said plunger rod to release the seal between the nose portion and the outlet opening of the syringe barrel wherein friction force maintains said at least one rib in an advanced position relative to a starting position within the syringe barrel such that the increased pressure causes any trapped fluid to be pushed through said outlet opening.