A method of preventing leakage of blood from a needle assembly comprising: a) receiving blood through a patient puncture tip and into a first chamber of a needle assembly, the needle assembly comprising: a needle housing defining a housing interior; a cannula (436, 452) having the patient puncture tip extending from a first end of the needle housing; a non-patient puncture tip extending from a second end of the needle housing, the non-patient puncture tip and the patient puncture tip being in fluid communication with each other through the cannula (436, 452); and a porous vent positioned within the housing interior and separating the housing interior into a first chamber and a second chamber, with the cannula (436, 452) being in fluid communication with the first chamber such that the sole communication path between the housing interior and the external environment is via the patient puncture tip, the porous vent including pores for passage of blood therethrough from the first chamber into the second chamber; b) establishing fluid communication between the non-patient puncture tip and an evacuated collection container, such that blood contained within the first chamber is drawn into the evacuated collection container and air is drawn out of the second chamber through the porous vent, thereby establishing a negative pressure gradient within the second chamber relative to the external environment of the needle assembly such that blood flows through the cannula into the first chamber and contacts the porous vent without sealing the porous vent; and c) drawing blood through the pores of the porous vent toward the second chamber based upon the negative pressure gradient established within the second chamber such that any blood contained within the cannula is displaced away from the patient puncture tip and toward the second chamber, wherein the receiving step a) comprises receiving blood through the patient puncture tip from a patient's bloodstream, and the drawing step c) is continued after removing the patient puncture tip from the patient's bloodstream thereby displacing blood away from the patient puncture tip after removing the patient puncture tip from the patient's bloodstream. a) receiving blood through a patient puncture tip and into a first chamber of a needle assembly, the needle assembly comprising: a needle housing defining a housing interior; a cannula (436, 452) having the patient puncture tip extending from a first end of the needle housing; a non-patient puncture tip extending from a second end of the needle housing, the non-patient puncture tip and the patient puncture tip being in fluid communication with each other through the cannula (436, 452); and a porous vent positioned within the housing interior and separating the housing interior into a first chamber and a second chamber, with the cannula (436, 452) being in fluid communication with the first chamber such that the sole communication path between the housing interior and the external environment is via the patient puncture tip, the porous vent including pores for passage of blood therethrough from the first chamber into the second chamber; a needle housing defining a housing interior; a cannula (436, 452) having the patient puncture tip extending from a first end of the needle housing; a non-patient puncture tip extending from a second end of the needle housing, the non-patient puncture tip and the patient puncture tip being in fluid communication with each other through the cannula (436, 452); and a porous vent positioned within the housing interior and separating the housing interior into a first chamber and a second chamber, with the cannula (436, 452) being in fluid communication with the first chamber such that the sole communication path between the housing interior and the external environment is via the patient puncture tip, the porous vent including pores for passage of blood therethrough from the first chamber into the second chamber; b) establishing fluid communication between the non-patient puncture tip and an evacuated collection container, such that blood contained within the first chamber is drawn into the evacuated collection container and air is drawn out of the second chamber through the porous vent, thereby establishing a negative pressure gradient within the second chamber relative to the external environment of the needle assembly such that blood flows through the cannula into the first chamber and contacts the porous vent without sealing the porous vent; and c) drawing blood through the pores of the porous vent toward the second chamber based upon the negative pressure gradient established within the second chamber such that any blood contained within the cannula is displaced away from the patient puncture tip and toward the second chamber,