Abstract: An apparatus for signalling an accidental loss of blood from a patient's fluid infusion line includes a sensor assembly for connection to the line, the assembly having a housing for receiving a portion of the line within a side opening, a holder having a clamp member for drawing together portions of the housing on opposite sides of the side opening for fixing the portion of the line within the opening relative to the housing, a source of radiation directed through the opening for intersecting the line, and a radiation detector in the housing for receiving radiation that is transmitted from the source through the line. The apparatus further includes a battery powered circuit, the circuit intermittently powering the radiation source, and producing a reference voltage, a train of signal pulses forming a sensor signal that is indicative of the radiation received by the detector and, when the sensor signal is in a predetermined relationship with the reference signal, an alarm signal.