A method and device for determining the concentration of blood constituents, in particular haemoglobin, in a hose line of an extracorporeal blood circuit of an extracorporeal blood treatment apparatus, and an extracorporeal blood treatment apparatus with a device for determining the concentration of a blood constituent, are based on the correction of the influence of the blood flow rate of the blood flowing through the hose line on the determination of the concentration of the blood constituent. The device comprises a computing and evaluation unit configured such that a correction factor is ascertained for the influence of the blood flow rate on the determination of the concentration of the blood constituent. The concentration of the blood constituent is then determined based on a relationship describing the dependence of the concentration of the blood constituent on the intensity of the decoupled electromagnetic radiation, taking account of the correction factor.
