Abstract: The device for detecting and/or determining the concentration of an analyte present in a tissue includes a sensor which is an optical fibre interferometer, and one interferometer arm being coated with an immobilised binding agent enabling selective binding of the analyte. The interferometer arm is mounted inside a guide enabling puncturing the tissue and performing an in situ measurement without the necessity to collect or prepare a sample. The guide is provided with a closed guide face, longitudinal perforations on sidewalls enabling the analyte to reach the binding agent, and an opening in the input end of the guide for introducing the interferometer with the arm into the guide. At the input end, the opening is sealed, enabling the isolation of the interior of the guide from the surroundings. The interferometer is mounted in a position in which the interferometer does not touch the inside walls of the guide.