An extracorporeal blood treatment apparatus comprising: - at least a main treatment unit comprising a first compartment and a second compartment separated by a semipermeable membrane, - a blood withdrawal line for blood to be treated connected to an inlet of the first compartment of the main treatment unit, - a blood return line connected to an outlet of the first compartment of the main treatment unit, wherein the blood return line, the blood withdrawal line and the first compartment are part of an extracorporeal blood circuit, - at least an auxiliary treatment unit comprising at least one fluid inlet and at least one fluid outlet, - a first fluid line having a first end connected to an outlet of the second compartment of the main treatment unit and a second end connected to the fluid inlet of the auxiliary treatment unit, - a second fluid line having a first end connected to the fluid outlet of the auxiliary treatment unit and a second end connected to said extracorporeal blood circuit, - a fluid flow regulator on at least one of said first and second fluid lines (12, 14), characterized in that the apparatus further comprises: - at least a sensor (18; 17; 34) for determining a parameter related to a pressure drop across said auxiliary treatment unit and emitting a corresponding pressure signal, and - a control unit configured to execute a control procedure comprising: ∘ receiving said pressure signal from the sensor and determining a corresponding parameter value, and ∘ controlling the flow regulator at least based on said parameter value. - at least a main treatment unit comprising a first compartment and a second compartment separated by a semipermeable membrane, - a blood withdrawal line for blood to be treated connected to an inlet of the first compartment of the main treatment unit, - a blood return line connected to an outlet of the first compartment of the main treatment unit, wherein the blood return line, the blood withdrawal line and the first compartment are part of an extracorporeal blood circuit, - at least an auxiliary treatment unit comprising at least one fluid inlet and at least one fluid outlet, - a first fluid line having a first end connected to an outlet of the second compartment of the main treatment unit and a second end connected to the fluid inlet of the auxiliary treatment unit, - a second fluid line having a first end connected to the fluid outlet of the auxiliary treatment unit and a second end connected to said extracorporeal blood circuit, - a fluid flow regulator on at least one of said first and second fluid lines (12, 14), characterized in that the apparatus further comprises: - at least a sensor (18; 17; 34) for determining a parameter related to a pressure drop across said auxiliary treatment unit and emitting a corresponding pressure signal, and - a control unit configured to execute a control procedure comprising: ∘ receiving said pressure signal from the sensor and determining a corresponding parameter value, and ∘ controlling the flow regulator at least based on said parameter value. ∘ receiving said pressure signal from the sensor and determining a corresponding parameter value, and ∘ controlling the flow regulator at least based on said parameter value.