Apparatus that detect an interruption of the connection between a blood treatment apparatus and a patient blood circulation which can be connected to the blood treatment apparatus via a connection means is described. The connection means can be attached to a fluid line. The apparatus includes a line shut-off and a pressure sensor which is configured to measure the fluid pressure prevailing in the fluid line at a position located between the line shut-off and the connection means. A control and evaluation unit is configured to evaluate the pressure or pressure curve occurring after blocking the line so as to detect an interruption or disturbance of the connection between the connection means and the patient blood circulation.
