The invention relates to a device and a method for identifying the direction of liquid flow through a dialyser (1), which has a blood chamber (4), through which blood flows, and a dialysis liquid chamber (3), through which dialysis liquid flows, which chambers are separated from one another by way of a semipermeable membrane (2). Moreover, the invention relates to an extracorporeal blood treatment device comprising a device for identifying the flow direction. A first aspect of the invention lies in measuring the clearance to identify the flow direction and comparing the measured clearance with a predetermined limit value, wherein a flow direction in the counterflow is deduced if the clearance is greater than the predetermined limit value. This aspect is based on the discovery that, during a blood treatment, clearance values above a specific limit value are no longer achievable in practice during an operation of the dialyser with the co-flow. A second aspect of the invention lies in measuring the clearance for identifying the flow direction and modifying the flowrate of the dialysis liquid. According to the second aspect, the flow direction is checked on the basis of the comparison between the measured change in the clearance and a calculated expected value of the change in the clearance for operation of the dialyser both with a counterflow and a co-flow. The invention according to the second aspect is based on the discovery that the magnitude of the relative change in the clearance in the case of a change in the dialysis liquid rate is always greater during operation with a co-flow than with a counterflow.
