An access disconnection system (10) is described. The system comprises a blood treatment machine (100) including a housing (104)； an access site detection housing (26) separate from the blood treatment machine housing； a transmitter (28) positioned in the access site housing and configured to transmit an ultrasound wave into a blood stream of a patient； a receiver (28, 34) positioned in the access site housing and configured to receive a reflection of the ultrasound wave from the blood stream of the patient； and electronic circuitry (40, 42, 44, 46, 52) in communication with the transmitter and the receiver and supported by the access site detection housing. The electronic circuitry is configured to send a wireless disconnection output to the blood treatment machine which is indicative of a change in the reflected ultrasound wave received by the receiver sufficient to expect that an access disconnection of one of the arterial line and the venous line has occurred.
