A medical sensing system including an access tube with a fluid infusate lumen and a sensing probe that can be passed through a second lumen in the access tube and extended into the vasculature for sensing a blood parameter. The sensing probe has a sensor on a distal end thereof and a shape that permits it to extend from the access tube such that the sensor is displaced laterally from the access tube, and out of the infusate flow. If the sensing probe passes through the lumen opening at the distal tip of the catheter, the distal end of the probe bends or otherwise deflects laterally so that the sensor is displaced laterally from the infusate flow path. Alternatively, the probe may exit a side port of the access tube upstream of an infusate side port. The deflectable probe is particularly useful for sensing glucose in blood through any number of conventional, off-the-shelf catheters.