A needle assembly preferably suitable for single sample blood collection and for determining vein entry when collecting a blood sample from a patient into a vacuum collection device. This assembly includes a housing with an interior chamber having a blood inlet needle on one side of the housing and a blood outlet needle on the other. The outlet needle extends interiorly into the chamber, is open at its distal end and includes a side hole therethrough. A vent plug covers the side hole by surrounding the outlet needle inside the chamber, while a sealant plug covers the open end of the outlet needle. This arrangement prevents liquid from flowing into the holes while allowing gas to flow through the vent plug into the side hole for escape from the assembly whereby the prevented liquid may then be viewed by a user through a transparent portion of the housing. The sealant plug is removable under the influence of a negative pressure gradient applied through the outlet needle so that this sealant plug is withdrawn into the outlet needle and then liquid is allowed to flow into the outlet needle for collection.