An extracorporeal blood treatment apparatus comprises at least a main treatment unit (1) comprising a first compartment (2) and a second compartment (3) separated by a semipermeable membrane (4), a blood withdrawal line (5), a blood return line (7), at least an auxiliary treatment unit (9) comprising at least one fluid inlet (10) and at least one fluid outlet (11), a first fluid line (12) having a first end connected to an outlet (13) of the second compartment (3) and a second end connected to the fluid inlet (10) of the auxiliary treatment unit (9), a second fluid line (14) having a first end connected to the fluid outlet (11) and a second end connected to said extracorporeal blood circuit, a fluid flow regulator (15) on one of said first and second fluid lines (12, 14), a sensor (18； 17； 34) for determining a parameter related to a pressure drop across the auxiliary treatment unit, and a control unit (16) configured to execute a control procedure comprising receiving the pressure signal and determining a corresponding parameter value, and for controlling the flow regulator (15) at least based on said parameter value.
