An extracorporeal circulation blood or treatment device comprising: a filter having a blood compartment and a dialysate compartment separated by a semipermeable membrane, said blood compartment having an inlet and an outlet, and said dialysate compartment having an inlet and an outlet; an arterial blood line adapted for transporting blood from the patient to the inlet of the blood compartment of the filter, having an arterial connection adapted for being connected to a patient, a peristaltic arterial blood pump and an arterial pressor sensor located between the arterial connection and the arterial blood pump; a venous blood line adapted for returning blood from the blood compartment of the filter, having a venous connection adapted for being connected to the patient a dialysate line for forwarding dialysate liquid from a dialysate source to the inlet of the dialysate compartment of the filter; a spent dialysate line for removing spent dialysate from the outlet of the dialysate compartment of the filter; and a controller, wherein said controller is connected to the arterial blood pump and the arterial pressor sensor, and wherein said controller is set up to receive signals from said arterial pressure sensor when the blood pump is running wherein the controller is set up to determine amplitude variation and frequency of the pressure signals received from the arterial pressure sensor, calculate a parameter value based on said amplitude variation and said frequency, and issue an alarm if the parameter value exceeds a pre-set threshold value, wherein the controller continuously determines the variance of amplitude change of the pressure signals received from the arterial pressure sensor, and said continuously determined variance is taken into account for calculating said parameter value, and characterized in that the controller is set up to determine if the frequency of the pressure signals from the arterial pressure sensor is the same as the occlusion frequency of the peristaltic arterial blood pump, and where this determination is taken into account when determining said parameter value. a filter having a blood compartment and a dialysate compartment separated by a semipermeable membrane, said blood compartment having an inlet and an outlet, and said dialysate compartment having an inlet and an outlet; an arterial blood line adapted for transporting blood from the patient to the inlet of the blood compartment of the filter, having an arterial connection adapted for being connected to a patient, a peristaltic arterial blood pump and an arterial pressor sensor located between the arterial connection and the arterial blood pump; a venous blood line adapted for returning blood from the blood compartment of the filter, having a venous connection adapted for being connected to the patient a dialysate line for forwarding dialysate liquid from a dialysate source to the inlet of the dialysate compartment of the filter; a spent dialysate line for removing spent dialysate from the outlet of the dialysate compartment of the filter; and a controller, wherein said controller is connected to the arterial blood pump and the arterial pressor sensor, and wherein said controller is set up to receive signals from said arterial pressure sensor when the blood pump is running wherein the controller is set up to determine amplitude variation and frequency of the pressure signals received from the arterial pressure sensor, calculate a parameter value based on said amplitude variation and said frequency, and issue an alarm if the parameter value exceeds a pre-set threshold value, wherein the controller continuously determines the variance of amplitude change of the pressure signals received from the arterial pressure sensor, and said continuously determined variance is taken into account for calculating said parameter value, and characterized in that the controller is set up to determine if the frequency of the pressure signals from the arterial pressure sensor is the same as the occlusion frequency of the peristaltic arterial blood pump, and where this determination is taken into account when determining said parameter value.