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 receiving a value for a first parameter, the first parameter being plasma conductivity, wherein said control unit is configured for setting a second parameter value for the dialysis fluid in the dialysis supply line at a set point, said second parameter of the dialysis fluid being at least one chosen in a group including a conductivity of the dialysis fluid, and a concentration of at least a substance in the dialysis fluid; characterized in that the setting of the second parameter value in the dialysis fluid includes the sub-step of calculating the parameter value as a function of a main contribution term based on the plasma conductivity and as a function of an adjustment contribution term based on a concentration of at least a substance in the dialysis fluid chosen in the group including bicarbonate, potassium, acetate, lactate, citrate, magnesium, calcium, sulphate, and phosphate, wherein the main contribution term is a dialysis fluid concentration of sodium at an isoconductive dialysis, the isoconductive dialysis being a dialysis treatment where the conductivity of the dialysis fluid does not change pre- to post-filtration unit and wherein the adjustment contribution term is the sodium concentration set point adjustment relative to the sodium concentration set point for the isoconductive dialysis and is calculated as a function of a difference between a first molar conductivity of a substance chosen in the group including sodium bicarbonate (NaHCO 3), sodium acetate (NaCH 3 COO), trisodium citrate (Na 3 C 6 H 5 O 7), sodium lactate (NaC 3 H 5 O 3), potassium chloride (KCl), and a molar conductivity of sodium chloride (NaCl), the adjustment contribution applied to the sodium concentration set point for the isoconductive dialysis provides a treatment chosen in the group including isotonic dialysis, isonatremic dialysis, isonatrikalemic dialysis and a treatment that removes from, or adds to, the plasma a defined amount of at least a substance. - 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 receiving a value for a first parameter, the first parameter being plasma conductivity, wherein said control unit is configured for setting a second parameter value for the dialysis fluid in the dialysis supply line at a set point, said second parameter of the dialysis fluid being at least one chosen in a group including a conductivity of the dialysis fluid, and a concentration of at least a substance in the dialysis fluid;