A system for determining the concentration of an analyte in a liquid sample comprising a detection unit for detecting light intensities which are radiated from subareas of a detection area of a test element as well as an evaluation unit which determines a frequency distribution for the detected light intensities wherein the frequency distribution has at least one first maximum caused by unwetted subareas or at least one reference area and a second maximum caused by wetted subareas and selects at least one light intensity on the basis of the frequency distribution and determines the concentration of the analyte from the at least one selected light intensity.