A needle assembly includes a transparent or translucent housing with a fluidinlet end, a fluid outlet end, a flashbackchamber, and a venting mechanism therebetween. Substantially axially alignedinlet and outlet cannulas extend from thehousing and communicate with the chamber. A sealable sleeve covers theexternal end of the outlet cannula. Relative volumes ofthe cannulas, the chamber, and the sleeve are selected to provide rapidreliable flashback indicative of venous entry with aninternal vent positioned within the housing so as to divide the interior intofirst and second chambers, with the second chamber beingadapted to maintain a negative pressure therein relative to the externalenvironment so as to inhibit leakage of blood from theneedle on withdrawal from the patient.
