An extracorporeal blood treatment apparatus comprises: a blood treatment device (2) comprising a blood chamber (3) and a fluid chamber (4) separated from one another by a semipermeable membrane (5)； an extracorporeal blood circuit (17) comprising a blood withdrawal line (6) connected to an inlet port (3a) of the blood chamber (3) and a blood return line (7) connected to an outlet port (3b) of the blood chamber (3)； a blood pump (21) configured to be coupled to the blood withdrawal line (6)； a hydraulic circuit (100) connectable to the fluid chamber (4), wherein the hydraulic circuit (100) comprises a fluid preparation device (9) connected to a water network (14) and configured to dilute concentrates in water to prepare a treatment fluid； a control unit (12) connected to the preparation device (9) and to the blood pump (21). The control unit (12) is configured to execute the following procedure: setting the hydraulic circuit (100) so that the fluid preparation device (9) bypasses the fluid chamber (4)； controlling the fluid preparation device (9) to prepare the treatment fluid while bypassing the fluid chamber (4)； and simultaneously controlling the blood pump (21) to perform pure ultrafiltration of a patient (P) connected to the extracorporeal blood circuit (17).
