An extracorporeal blood treatment apparatus is provided comprising a filtration unit (2) connected to a blood circuit (17) and to a dialysis fluid circuit (32), a preparation device (9) for preparing and regulating the composition of the dialysis fluid a control unit (12) is configured for determining or receiving a proposed value (prop.value Cprop.value) of a sodium concentration for the dialysis fluid in the dialysis supply line (8) and to determine a set value (fsetvalue Cse tvalue) for the sodium concentration in the dialysis fluid as a function of the proposed value (prop.value Cprop.value). For at least an interval of proposed values for the sodium concentration, the control unit (12) is configured to determine the set value so that the set value is different from the proposed value (prop.value cprop.value) and so that distinct set values are determined from distinct proposed values. The set value is biased towards a predetermined pivot value that represents e.g. the average sodium concentration in the blood of a population and may not be outside a reference range.
