An apparatus for extracorporeal treatment of blood (1) comprising a treatment unit (2), a blood withdrawal line (6), a blood return line (7), a preparation line (19) and a spent dialysate line (13). A control unit (10) is configured to calculate values of a parameter relating to treatment effectiveness based on measures of the conductivity in the spent dialysate line (13) subsequent to an alternating conductivity perturbation continuously imposed on the preparation line (19) of fresh dialysis fluid.
