A method for determining the concentration of an analyte in a biological sample comprising the steps of: (1) providing an optical measuring instrument comprising a thermally controllable optical measuring element that comes into contact with the surface of the biological sample; (2) applying a coupling agent to the optical measuring element or to the surface of the biological sample or to both so that the coupling agent will be disposed at the interface of the surface of the biological sample and the optical measuring element; (3) measuring optical properties of the biological sample by means of the optical measuring instrument; and (4) correlating the optical properties of the biological sample with the concentration of the analyte in the biological sample. The coupling agent can be selected from the group consisting of silicone oil, mineral oil, polyethylene glycols, and oils from natural resources.