A device and method for monitoring an access to a patient, an extracorporeal blood circuit and/or a dialysing fluid system includes a centrifugal pump for conveying blood or dialysing fluid instead of an occluding pump. Centrifugal pumps bring about a large change in flow rate by even a small change in pressure difference across the pump. The device includes a measuring unit for measuring the flow rate of blood or dialysing fluid conveyed by the centrifugal pump, and a control and computing unit configured to determine an incorrect vascular access or malfunction if a change in measured flow rate Q is more than a predetermined amount. For example, a small drop in pressure in the venous blood line leads to a marked increase in the flow rate of the centrifugal pump, which is used as a basis for the detection of an incorrect vascular access.