The invention provides a method for the determination of the state of an entity, in particular the state of health of a human being or animal. The method includes the step of contacting secreted compounds or related reaction products with a surface linked to an optical waveguide in such a way that compounds binding to the surface can influence the propagation of light in the waveguide, determining the occurring influence by means of an optical interferometric measuring principle, and deducing the state from the occurring influence. The invention further provides a device for the determination of such a state.
