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 concentration of at least a substance for the dialysis fluid, wherein the control unit is configured for either calculating the proposed value (Cond prop) for the first parameter or receiving the proposed value (Cond prop) as an input, • obtaining a value for a second parameter, the second parameter being related to a patient fluid overload; • 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 second parameter, characterized in that said proposed value (Cond prop) for the first parameter is the substance concentration set point or the conductivity set point for running an isotonic dialysis or isonatremic dialysis or an isonatrikalemic dialysis, and the control unit is configured to determine the set value (Cond set) for the first parameter as a linear function of the second parameter and as an algebraic sum of at least one first term and one second term, the first term being equal to the proposed value (Cond prop), the second term being function of at least the second 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 concentration of at least a substance for the dialysis fluid, wherein the control unit is configured for either calculating the proposed value (Cond prop) for the first parameter or receiving the proposed value (Cond prop) as an input, • obtaining a value for a second parameter, the second parameter being related to a patient fluid overload; • 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 second 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 concentration of at least a substance for the dialysis fluid, wherein the control unit is configured for either calculating the proposed value (Cond prop) for the first parameter or receiving the proposed value (Cond prop) as an input, • obtaining a value for a second parameter, the second parameter being related to a patient fluid overload; • 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 second parameter, characterized in that said proposed value (Cond prop) for the first parameter is the substance concentration set point or the conductivity set point for running an isotonic dialysis or isonatremic dialysis or an isonatrikalemic dialysis, and the control unit is configured to determine the set value (Cond set) for the first parameter as a linear function of the second parameter and as an algebraic sum of at least one first term and one second term, the first term being equal to the proposed value (Cond prop), the second term being function of at least the second parameter.