A monitor for an extracorporeal therapy access site is disclosed. The monitor includes a bandage atop or adjacent the access site and a sensor for monitoring the bandage. The bandage includes a layer of polymer that expands when wetted with blood. The expansion causes a break in continuity of the sensor, or in an alternate embodiment, causes a sensor to cease detecting a target. When the break occurs, the control circuit monitoring the bandage sends a signal that a break has occurred, and a remote monitor then takes appropriate action, such as ceasing therapy, sending an alert, or sounding an alarm. In another embodiment, connecting wires in a continuity circuit are held apart by a polymer that dissolves when contacted by blood. If a leak occurs and a small portion of the polymer dissolves, the wires make contact, thus detecting a blood leak.