Methods and devices for analyzing body fluids, in particular for determining blood glucose. Body fluid is applied to a test element for single-use in a test device and the test element is optically coupled to a photometric measuring unit by means of an optical transmission system, wherein a time course of measuring values is recorded after the body fluid has been applied in order to detect a component of the body fluid on the test element. The transmission behavior of the optical transmission system may be controlled by recording measuring values at two different measurement wavelengths.