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).