A device sets up a dilution measurement site on a hemodialyzer for determining the filling status of a dialysis patient during dialysis treatment, by dilution measurements. The arterial connection piece is connected with an arterial fistula needle, and the venous connection piece is connected with a venous fistula needle. The venous connection piece has an injection channel through which a bolus required to perform a dilution measurement can be injected into the blood, which flows through the venous connection piece coming from the hemodialyzer, in the direction of the venous blood vessel access. The arterial connection piece has a sensor measuring the temperature of the blood flowing through the venous connection piece coming from the arterial blood vessel access, in the direction of the hemodialyzer.
