The invention relates to a method and a device for determining the concentration of a blood constituent in a tube, in particular a tube (5) of an extracorporal blood circuit (I) of an extracorporal blood treatment unit. The invention also relates to an extracorporal blood treatment device comprising a device (11) for determining the concentration of a blood constituent. Said claimed method and the claimed device relate to the correction of the influences of the tube for determining the concentration of the blood constituent. In order to determine the correction factor k, a reference measurement is carried out with a liquid other than blood, in order to be able to determine the correlation between the intensity of emitted electromagnetic radiation in a tube which is filled with blood containing the blood constituent in a defined concentration, and in a tube which is filled with a liquid other than blood. The determination of the correction factor then takes place based on the previously determined correlation between the intensity of the emitted electromagnetic radiation in a tube filled with blood and in the tube filled with a liquid other than blood and based on the previously measured intensity of the electromagnetic radiation which is emitted by the tube filled with a liquid other than blood.
