The invention relates to a device and a method for detecting the direction of fluid flow through a dialyser 1 which comprises a blood chamber 4, through which blood flows, and a dialysate chamber 3, through which dialysate flows, which are separated from one another by a semi-permeable membrane 2. In addition, the invention relates to an extracorporeal blood treatment device comprising a device for detecting the flow direction. A first aspect of the invention is to measure the clearance in order to detect the flow direction and to compare the measured clearance with a specified limit value, a flow direction in countercurrent flow being concluded if the clearance is greater than the specified limit value. This aspect is based on the finding that in the case of blood treatment in practice with operation of the dialyser in co-current flow, clearance values above a certain limit value can no longer be achieved. A second aspect of the invention is to measure the clearance to detect the flow direction and to change the flow rate of the dialysate. Checking the flow direction according to the second aspect is based on the comparison of the measured change in clearance with a calculated expected value of the change in clearance for operation of the dialyser in both countercurrent flow and co-current flow. The invention according to the second aspect is based on the finding that the amount of the relative change in clearance in the event of a change in dialysate rate is always greater in the case of operation in co-current flow than in countercurrent flow.
