The invention relates to a device and a method for monitoring an access to a patient,in particular a vascular access in extracorporeal blood treatment, in whicha patients blood is withdrawn from the patient via an arterial hose line(6) and is returned to the patient via a venous hose line (7). In the device accordingto the invention and in the method according to the invention, an alternatingvoltage signal, referred to a common ground potential, is coupled in and out ofthe arterial and venous hose lines (6, 7), and the blood flowing through the arterialand venous hose lines is at ground potential. In this way, disturbances, whichcan be attributed particularly to movements of the hose lines, are reduced.
