Abstract: A method and assembly for inserting one or more diagnostic sensors into a biological fluid vessel for diagnosing the condition of the vessel or fluid. Different types of diagnostic sensors, such as pressure, velocity, pH, temperature, or infusion catheter, are particularly adapted for use with a steerable guidewire for selective positioning in the vessel. In one application, the method is used in coronary diagnosis, to determine the pressure gradient or other fluid characteristics across a coronary valve or stenosis in the coronary arterial tree. In this method, a guiding catheter is percutaneously inserted with its distal end proximate the ostium. A guidewire is inserted through the guiding catheter and subselective positioned in a region of interest in the coronary arterial tree. Multiple diagnostic sensors are then threaded down the guidewire and positioned in the region of interest.