Abstract: A method and an apparatus for determining the content of a constituent of blood of an individual are disclosed. In the method a whole blood stream is extracted from a blood vessel of said individual, the stream being directed through a path defining a substantially non-varying flow-through area wherein is provided a flow-through measuring cuvette including opposite first and second optical transparent surface parts defining an optical transmission path of the order of 0.5-2.0 mm. The whole blood stream flowing through the measuring cuvette is irradiated by irradiating the first optical transparent surface part of the measuring cuvette with multi-wavelength near infrared light. The near infrared absorption spectrum is detected and the content of the constituent is quantified on the basis of the detected near infrared absorption data. The method is particularly suited for measuring constituents of whole blood in an extracorporeal loop, for example in hemodialysis.