The invention relates to a device (25) and a method for recognizing a cause of a deviation from an ideal operating state or an ideal treatment course in an extra corporeal blood treatment. In the extra corporeal blood treatment according to the invention the blood to be treated flows in an extra corporeal blood circuit (9) through a blood chamber (3) of a dialyzer (1) divided by a semipermeable membrane (2) into the blood chamber (3) and a dialysis liquid chamber (4) and dialysis liquid in a dialysis liquid circuit (10) flows through the dialysis liquid chamber (4) of the dialyzer. The device according to the invention has the following: a means for changing a physical or chemical parameter (170 171) of the dialysis liquid upstream of the dialysis liquid chamber (1) during the blood treatment a measuring means (174) for measuring a physical or chemical parameter of the dialysis liquid downstream of the dialysis liquid chamber a device for determining a value of the dialysance or clearance from a change in the measured physical or chemical parameter at a specific point in time during the blood treatment a device (25) for detecting a deviation of the determined value of dialysance or clearance from a reference value of the dialysance or clearance and an evaluation device (25) for recognizing a cause of a deviation from an ideal operating state in dependence on the specific point in time.
