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 ; - 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, characterized by the fact that the apparatus further comprises: - a control unit connected to the regulating means and programmed for receiving a desired mass transport of a substance at the end of a treatment session and a value representative of a first parameter, the first parameter being chosen in the group including a plasma conductivity, a concentration of the substance in the blood, a conductivity of the dialysis fluid, a concentration of the substance in the dialysis fluid, 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, a concentration of the substance in the dialysis fluid; wherein the control unit is configured to calculate the second parameter value as a function of a main contribution term, based on the first parameter, and as a function of an adjustment contribution term based on the desired mass transport, the main contribution term being a dialysis fluid concentration of a substance at a dialysis treatment where the substance concentration of the dialysis fluid does not change pre-to post- filtration unit . - 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 ; - 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 desired mass transport of a substance at the end of a treatment session and a value representative of a first parameter, the first parameter being chosen in the group including a plasma conductivity, a concentration of the substance in the blood, a conductivity of the dialysis fluid, a concentration of the substance in the dialysis fluid, 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, a concentration of the substance in the dialysis fluid; <EOS>