A catheter assembly including an elongate catheter tube is disclosed. The number of lumens defined by the catheter tube can vary as a function of longitudinal length along the catheter tube. For instance, the catheter tube can define three lumens from the proximal end of the catheter tube and terminate one of the lumens at an intermediate termination point such that only two lumens are defined further distally along the catheter tube. A sensor can be placed in the terminating lumen so as to isolate it from the other lumens and from blood or other body fluids while still enabling the sensor to reside within the patient body when the catheter tube is positioned in the patient for use. In addition to this, various other lumen transition and sensor configurations are disclosed.
