Renal failure therapy system includes a blood filter, a blood pump, a dialysis fluid pump, a blood line connecting to a patient, a transmitter for receiving a transmit signal and transmitting a wave into blood flowing through the blood line, a receiver for receiving the wave and emitting a corresponding received signal, and a processor for initiating the transmit signal and for receiving the received signal, configured to convert the transmit and received signals into a frequency domain, extract spectral values for the converted transmit and received signals, calculate at least one of a reflection coefficient (R) or an impedance ratio (I) using the spectral values, and analyze an imaginary part of at least one of R or I to determine if an access disconnection event concerning fluid communication between the blood line and the patient has occurred.
