A device (25) and a method for detecting the cause of a deviation from an ideal operating state of an extracorporeal blood treatment, wherein blood to be treated in an extracorporeal blood circulation (9) flows through the blood chamber (3) of a dialyzer (1), subdivided by a semipermeable membrane (2) into a blood chamber (3) and a dialysis fluid chamber (4), and dialysis fluid flows through the dialysis fluid chamber (4). The method includes the following: changing a characteristic (170； 171) of the dialysis fluid upstream from the dialysis fluid chamber (1), measuring a characteristic of the dialysis fluid downstream from the dialysis fluid chamber, determining a dialysance or clearance at a certain point in time during the blood treatment, detecting a deviation in the dialysance or clearance, and detecting the cause of a deviation from an ideal operating state as a function of the certain point in time.
