During a dialysis treatment carried out by a dialysis apparatus, in which a dialysis liquid is circulated to a clean dialysate filter (43) arranged before a dialyser (3), the conductivity of the dialysis liquid is altered according to an altering function that is a function of a mathematical model of the filter (43), in a way that will make the conductivity after the filter change more quickly. A conductivity cell (47), which is arranged after the dialyser, measures the conductivity of the spent dialysis liquid changed in response to said alteration. A control and computer system (52) evaluates the dialysance from said measured response. The conductivity can be changed for a comparatively short time period that is sufficient to allow reliable results.