An apparatus for extracorporeal blood treatment comprising: - a filtration unit having a primary chamber and a secondary chamber separated by a semi-permeable membrane; - a blood withdrawal line connected to an inlet of the primary chamber, - a blood return line connected to an outlet of the primary chamber, said blood lines being configured for connection to a patient cardiovascular system; - a dialysis supply line connected to an inlet of the secondary chamber; - a dialysis effluent line connected to an outlet of the secondary chamber; - a preparation device for preparing a dialysis fluid connected to said supply line and comprising regulating means for regulating the composition of the dialysis fluid, - a control unit connected to the regulating means and programmed for: • obtaining a proposed value (Cond prop) of a first parameter for the dialysis fluid in the dialysis supply line, the first parameter being chosen in the group including a conductivity for the dialysis fluid, a conductivity-related parameter for the dialysis fluid, a concentration of at least one ionic substance for the dialysis fluid, a concentration-related parameter of at least one ionic substance for the dialysis fluid, • obtaining a value for a third parameter, the third parameter being chosen in the group including: ∘ a non-ionic substance concentration in the patient; o a concentration-related parameter of at least a non-ionic substance in the patient; • determining a set value (Cond set) for the first parameter as a function of the proposed value (Cond prop) for the first parameter and the third parameter. - a filtration unit having a primary chamber and a secondary chamber separated by a semi-permeable membrane; - a blood withdrawal line connected to an inlet of the primary chamber, - a blood return line connected to an outlet of the primary chamber, said blood lines being configured for connection to a patient cardiovascular system; - a dialysis supply line connected to an inlet of the secondary chamber; - a dialysis effluent line connected to an outlet of the secondary chamber; - a preparation device for preparing a dialysis fluid connected to said supply line and comprising regulating means for regulating the composition of the dialysis fluid, - a control unit connected to the regulating means and programmed for: • obtaining a proposed value (Cond prop) of a first parameter for the dialysis fluid in the dialysis supply line, the first parameter being chosen in the group including a conductivity for the dialysis fluid, a conductivity-related parameter for the dialysis fluid, a concentration of at least one ionic substance for the dialysis fluid, a concentration-related parameter of at least one ionic substance for the dialysis fluid, • obtaining a value for a third parameter, the third parameter being chosen in the group including: ∘ a non-ionic substance concentration in the patient; o a concentration-related parameter of at least a non-ionic substance in the patient; • determining a set value (Cond set) for the first parameter as a function of the proposed value (Cond prop) for the first parameter and the third parameter. • obtaining a proposed value (Cond prop) of a first parameter for the dialysis fluid in the dialysis supply line, the first parameter being chosen in the group including a conductivity for the dialysis fluid, a conductivity-related parameter for the dialysis fluid, a concentration of at least one ionic substance for the dialysis fluid, a concentration-related parameter of at least one ionic substance for the dialysis fluid, • obtaining a value for a third parameter, the third parameter being chosen in the group including: ∘ a non-ionic substance concentration in the patient; o a concentration-related parameter of at least a non-ionic substance in the patient; ∘ a non-ionic substance concentration in the patient; o a concentration-related parameter of at least a non-ionic substance in the patient; • determining a set value (Cond set) for the first parameter as a function of the proposed value (Cond prop) for the first parameter and the third parameter.