Apparatus and control methods for determining a distribution volume of at least one uremic substance in a dialysis patient are disclosed. The apparatus may include a dialyzer, an optional flow rate measuring system for detecting the flow rate of a dialysis fluid through the dialyzer, a dialysis fluid measuring system for detecting at least one dialysis fluid parameter that depends on the at least one substance in the dialysis fluid, and a computer for establishing a dialysis dose based on the dialysis fluid parameter and for establishing the distribution volume based on the dialysis dose taking a clearance factor and dialysis duration into account.
