A method as well as an apparatus is disclosed for determining the efficiency of a currently performed kidney replacement therapy on the dialysis side making use of a dialysis machine which in a first step is operated in a hemodialysis or hemodiafiltration process and in a second step is sequentially changed over to a hemofiltration process or is changed over to a sequential mode in which merely a flow from the blood compartment via the semipermeable membrane to the dialysis fluid compartment of the dialyser is generated in which, according to a third step, a sensor for determining or measuring the concentration at least of uremic toxins in the saturated dialysate or ultra-filtrate which is connected downstream of a dialyser on the dialysis side outputs corresponding measuring signals that are representative of the current concentration at least of uremic toxins in the blood to a calculation or determination unit.
