In a method for the measurement of the concentration of a substance in a liquid, especially for blood sugar measurement, the liquid to be measured is applied to the measuring field of a test strip, which measuring field is composed of a hydrophilic material, and the change of the optical reflectivity or transmissivity effected thereby in the area of the measuring field is captured. A measured value taken after a pre-given time is compared with a reference value and an indication is made if a relationship of the measured value to the reference value exceeds a pre-given threshold value.