Abstract: A dislodgement sensor for use with dialysis and other blood transfusion equipment that detects when a venous needle is dislodged from a patient. The sensor includes a photosensor attached to the venous needle or to tubing attached to the venous needle, an opaque cover placed over but not attached to the photosensor, and a signal line from the photosensor to the equipment. If the venous needle is dislodged, the photosensor is withdrawn from beneath the opaque cover, exposing the photosensor to light, which causes the photosensor to send a signal to the equipment indicating that the venous needle has been dislodged. Also, a method of using such a sensor.