Abstract: An extracorporeal blood treatment machine includes a dialyzer and a sensor device downstream of the dialyzer on a dialysis fluid side. The machine is connected to a control and computing unit configured to qualitatively and quantitatively determine at least one preferably selected or selectable blood component in the used dialysis fluid. The control and computing unit is adapted to put the machine into a mode in which a dialysis fluid amount is confined within the dialyzer at least until the concentrations of the blood component on the dialysis fluid side and on the blood side of the dialyzer are in equilibrium, and thereupon to switch the machine into a mode in which the dialysis fluid flow is permitted to leave the dialyzer to feed the confined dialysis fluid amount as a dialysis fluid bolus to the sensor device for determining the blood component concentration contained in the bolus.