Abstract: A method for determining a substance concentration in a particle-containing liquid, in particular of glucose in blood, wherein a refractive index of the liquid is dependent on a concentration of the substance dissolved therein, includes emitting a measuring light beam of at least one wavelength onto a sample containing the liquid. The method also includes detecting a first light component scattered by scattering on the particles contained in the liquid at a first angle, which corresponds in particular to a forward scattering of the light component. The method further includes detecting a second light component scattered at a second angle by the particles contained in the liquid. The method additionally includes determining a concentration or a proportion of the substance in the liquid from the detected first and second light components, in particular by forming a ratio from the detected first and second light components.