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 including at least an infusion line connected to said blood circuit; - a dialysis effluent line connected to an outlet of the secondary chamber; - a control unit configured to run at least a hemofiltration treatment or a hemodiafiltration treatment, each of said treatment including an infusion of substitution fluid through said infusion line, the control unit being programmed for receiving a value of a first parameter representative of an isoconductive dialysis, the first parameter being chosen in the group including a concentration of at least a substance, a concentration related parameter of at least a substance, a conductivity or a conductivity related parameter, - 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, the regulating means being connected to the control unit, characterized in that said control unit is configured for: - calculating the value of a second parameter of the blood, said second parameter being a concentration of at least a substance in the blood; wherein the step of calculating the value of the second parameter is performed as a function of a main contribution term based on the first parameter and as a function of an offset contribution term based on a difference in concentration of at least a substance in the dialysis fluid and the same substance in the plasma, wherein said concentration of the substance in the plasma is estimated and, for HDF pre-dilution treatment and HF pre-dilution treatment, said estimated plasma concentration is a diluted plasma concentration, said substance in the dialysis fluid being chosen in the group including bicarbonate, potassium, acetate, lactate and citrate; - setting a third parameter value for the dialysis fluid in the dialysis supply line at a set point based on the second parameter, said third parameter of the dialysis fluid being at least one chosen in a group including a conductivity of the dialysis fluid, a conductivity-related parameter of the dialysis fluid, a concentration of at least a substance in the dialysis fluid and a concentration-related parameter of at least a substance in the dialysis fluid; and - driving the regulating means for regulating the conductivity of the dialysis fluid or the concentration of at least a substance in the dialysis fluid at the set point. - 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 including at least an infusion line connected to said blood circuit; - a dialysis effluent line connected to an outlet of the secondary chamber; - a control unit configured to run at least a hemofiltration treatment or a hemodiafiltration treatment, each of said treatment including an infusion of substitution fluid through said infusion line, the control unit being programmed for receiving a value of a first parameter representative of an isoconductive dialysis, the first parameter being chosen in the group including a concentration of at least a substance, a concentration related parameter of at least a substance, a conductivity or a conductivity related parameter, - 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, the regulating means being connected to the control unit, characterized in that said control unit is configured for: - calculating the value of a second parameter of the blood, said second parameter being a concentration of at least a substance in the blood; wherein the step of calculating the value of the second parameter is performed as a function of a main contribution term based on the first parameter and as a function of an offset contribution term based on a difference in concentration of at least a substance in the dialysis fluid and the same substance in the plasma, wherein said concentration of the substance in the plasma is estimated and, for HDF pre-dilution treatment and HF pre-dilution treatment, said estimated plasma concentration is a diluted plasma concentration, said substance in the dialysis fluid being chosen in the group including bicarbonate, potassium, acetate, lactate and citrate; - setting a third parameter value for the dialysis fluid in the dialysis supply line at a set point based on the second parameter, said third parameter of the dialysis fluid being at least one chosen in a group including a conductivity of the dialysis fluid, a conductivity-related parameter of the dialysis fluid, a concentration of at least a substance in the dialysis fluid and a concentration-related parameter of at least a substance in the dialysis fluid; and - driving the regulating means for regulating the conductivity of the dialysis fluid or the concentration of at least a substance in the dialysis fluid at the set point.