Extracorporeal blood treatment apparatus, comprising: a blood treatment device; an extracorporeal blood circuit (3, 5) coupled to the blood treatment device; a blood pump configured to be coupled to a pump section of the extracorporeal blood circuit (3, 5); a fluid circuit (8; 12; 15; 17; 18; 22; 41; 42; 44) operatively connected to the extracorporeal blood circuit (3, 5) and at least a fluid pump (11; 13; 19; 23; 37; 39) configured to be coupled to the fluid circuit (8; 12; 15; 17; 18; 22; 41; 42; 44); wherein the extracorporeal blood circuit (3, 5) and/or the fluid circuit (8; 12; 15; 17; 18; 22; 41; 42; 44) present/s a connecting zone configured to be connected to at least a soft bag (30, 24; 33, 34) respectively for blood or for a fluid; at least a pressure sensor (25, 26) configured to measure at least a pressure (P1, P2, Pwdr, Pret) in the extracorporeal blood circuit (3, 5) or in the fluid circuit (22; 41, 42, 44); a control unit configured to check the connection of the soft bag (30, 24; 33, 34) to the extracorporeal blood circuit (3, 5) or to the fluid circuit (22; 41, 42, 44) through the following procedure: sucking a medium from the connecting zone through the blood pump or through the fluid pump (11; 13; 19; 23; 37; 39); measuring at least a pressure trend (P1, P1 - P2, P2 - P1, Pwdr, Pwdr - Pret, Pret - Pwdr) over time in the extracorporeal blood circuit (3, 5) or in the fluid circuit (8; 12; 15; 17; 18; 22; 41; 42; 44) through said at least a pressure sensor (25, 26); establishing from said measured pressure trend (P1, P1 - P2, P2 - P1, Pwdr, Pwdr - Pret, Pret - Pwdr) if the soft bag (30, 24; 33, 34) is connected to the extracorporeal blood circuit (3, 5) or to the fluid circuit (22; 41, 42, 44) at the connecting zone. a blood treatment device; an extracorporeal blood circuit (3, 5) coupled to the blood treatment device; a blood pump configured to be coupled to a pump section of the extracorporeal blood circuit (3, 5); a fluid circuit (8; 12; 15; 17; 18; 22; 41; 42; 44) operatively connected to the extracorporeal blood circuit (3, 5) and at least a fluid pump (11; 13; 19; 23; 37; 39) configured to be coupled to the fluid circuit (8; 12; 15; 17; 18; 22; 41; 42; 44); wherein the extracorporeal blood circuit (3, 5) and/or the fluid circuit (8; 12; 15; 17; 18; 22; 41; 42; 44) present/s a connecting zone configured to be connected to at least a soft bag (30, 24; 33, 34) respectively for blood or for a fluid; at least a pressure sensor (25, 26) configured to measure at least a pressure (P1, P2, Pwdr, Pret) in the extracorporeal blood circuit (3, 5) or in the fluid circuit (22; 41, 42, 44); a control unit configured to check the connection of the soft bag (30, 24; 33, 34) to the extracorporeal blood circuit (3, 5) or to the fluid circuit (22; 41, 42, 44) through the following procedure: sucking a medium from the connecting zone through the blood pump or through the fluid pump (11; 13; 19; 23; 37; 39); measuring at least a pressure trend (P1, P1 - P2, P2 - P1, Pwdr, Pwdr - Pret, Pret - Pwdr) over time in the extracorporeal blood circuit (3, 5) or in the fluid circuit (8; 12; 15; 17; 18; 22; 41; 42; 44) through said at least a pressure sensor (25, 26); establishing from said measured pressure trend (P1, P1 - P2, P2 - P1, Pwdr, Pwdr - Pret, Pret - Pwdr) if the soft bag (30, 24; 33, 34) is connected to the extracorporeal blood circuit (3, 5) or to the fluid circuit (22; 41, 42, 44) at the connecting zone. sucking a medium from the connecting zone through the blood pump or through the fluid pump (11; 13; 19; 23; 37; 39); measuring at least a pressure trend (P1, P1 - P2, P2 - P1, Pwdr, Pwdr - Pret, Pret - Pwdr) over time in the extracorporeal blood circuit (3, 5) or in the fluid circuit (8; 12; 15; 17; 18; 22; 41; 42; 44) through said at least a pressure sensor (25, 26); establishing from said measured pressure trend (P1, P1 - P2, P2 - P1, Pwdr, Pwdr - Pret, Pret - Pwdr) if the soft bag (30, 24; 33, 34) is connected to the extracorporeal blood circuit (3, 5) or to the fluid circuit (22; 41, 42, 44) at the connecting zone.