A closed blood sampling system within a pressure monitoring line having a control valve that enables a clearance reservoir to be isolated from the pressure column when no samples are being taken. The valve is a stopcock-like device that includes a rotating valve member and attached control handle with clear visible and tactile indicators for the mode of operation. The rotating valve member has a number of internal and circumferential channels for connecting or disconnecting select ports in the core of the valve. By isolating the clearance reservoir, the quality of the pressure signal is improved such that the sampling line can be lengthened for greater convenience in the intensive care or operating room. The valve may also incorporate a blunt cannula sampling site therewithin.