Abstract: A fluid collection assembly (10) and method for use thereof is provided including a fluid collection cartridge (20) having a tubular member (21) with a plunger assembly slidably inserted. The plunger assembly includes a stopper (32) and a plunger rod (31). The stopper and plunger rod are removably interlocked together which enables the plunger rod to exert a force in a distal direction on the stopper and to be removed from the stopper upon the application of a proximal force thereto. The fluid collection assembly is particular useful in the collection of arterial blood. In use, the assembly is primed with a liquid anticoagulant, the plunger rod is removed, and a blood sample is collected with the arterial pressure causing the stopper to travel in a proximal direction along the cartridge.