An apparatus for extracorporeal blood treatment, comprising: - an infusion control arrangement arranged for infusing a pre-dilution flow rate upstream a blood treatment unit and a post-dilution flow rate downstream the blood treatment unit ; - an ultrafiltration, UF, arrangement arranged for ultrafiltration of a liquid through a semipermeable membrane of the blood treatment unit ; characterized in that the apparatus comprises a control unit configured to: ∘ configure the infusion control arrangement to obtain a plurality of different configurations of the pre-dilution flow rate and/or post-dilution flow rate, wherein the plurality of different configurations comprises different ratios of pre-dilution flow rate to post-dilution flow rate, or different pre-dilution flow rates, and for each of the plurality of different configurations: ▪ change an operating situation of the UF arrangement which entails a change of transmembrane pressure (TMP); ▪ detect a plurality of ultrafiltration flow rate values through the membrane as a function of the TMP, on changing said operating situation; ▪ evaluate an optimal ultrafiltration flow rate value from a comparison of the detected ultrafiltration flow rates; and ▪ estimate a clearance value for a certain solute at the optimal ultrafiltration flow rate value; ∘ evaluate a preferred configuration of the pre-dilution flow rate and/or post-dilution flow rate from a comparison of the corresponding estimated clearance values. - an infusion control arrangement arranged for infusing a pre-dilution flow rate upstream a blood treatment unit and a post-dilution flow rate downstream the blood treatment unit ; - an ultrafiltration, UF, arrangement arranged for ultrafiltration of a liquid through a semipermeable membrane of the blood treatment unit ; ∘ configure the infusion control arrangement to obtain a plurality of different configurations of the pre-dilution flow rate and/or post-dilution flow rate, wherein the plurality of different configurations comprises different ratios of pre-dilution flow rate to post-dilution flow rate, or different pre-dilution flow rates, and for each of the plurality of different configurations: ▪ change an operating situation of the UF arrangement which entails a change of transmembrane pressure (TMP); ▪ detect a plurality of ultrafiltration flow rate values through the membrane as a function of the TMP, on changing said operating situation; ▪ evaluate an optimal ultrafiltration flow rate value from a comparison of the detected ultrafiltration flow rates; and ▪ estimate a clearance value for a certain solute at the optimal ultrafiltration flow rate value; ▪ change an operating situation of the UF arrangement which entails a change of transmembrane pressure (TMP); ▪ detect a plurality of ultrafiltration flow rate values through the membrane as a function of the TMP, on changing said operating situation; ▪ evaluate an optimal ultrafiltration flow rate value from a comparison of the detected ultrafiltration flow rates; and ▪ estimate a clearance value for a certain solute at the optimal ultrafiltration flow rate value; ∘ evaluate a preferred configuration of the pre-dilution flow rate and/or post-dilution flow rate from a comparison of the corresponding estimated clearance values. <EOS>