The invention relates to an extracorporeal blood treatment apparatus comprising a blood treatment unit 1 that is divided by a semipermeable membrane 2 into a first compartment 3, which is part of a fluid system II, and a second compartment 4, which is part of an extracorporeal blood circuit I. The invention also relates to a method for operating a blood treatment apparatus of this kind. The blood treatment apparatus according to the invention has a pressure-based checking device 32 which interacts with the control unit 31 for a valve device 21 and is designed such that a fluid connection between an upstream portion 20A and a downstream portion 20B of a flow path 20 for a special operating mode can only be established if the pressure-based checking device 32 detects an operating state in which it is ensured that fluid in the flow path 20 for a special operating mode flows towards a flow path 10 leading to a drain 11. This ensures that the fluid in question can only flow into the flow path 10 that leads to the drain 11, and cannot get into another flow path 8 in which fresh treatment fluid is located.

The current invention relates to a peritoneal dialysis filter device, which comprises a housing comprising a first port and a second port, and a hollow fibre membrane formed from hollow hydrophilic fibres within the housing. When in use, a dialysate from a subject enters the filter device through the first port and exits via the second port in an outflow direction, and a regenerated dialysate from a sorbent system enters the filter device through the second port and exits via the first port in an inflow direction. Also disclosed herein is a peritoneal dialysis system comprising said filter device, a method for controlling dialysate flow in a peritoneal dialysis system, and a haemodialysis device.