An implantable sensor 1, comprising a hydrogel 11, a glucose-binding protein 12 and a reference molecule 13, whose binding affinity for glucose differs from the binding affinity for glucose of the glucose-binding protein at least by a factor of 10, wherein the glucose-binding protein and the reference molecule change their electromagnetic behaviour and/or their fluorescence behaviour on binding glucose. Also, an electrical patch 2, comprising a measuring element 21 and a first communication element 22 for wireless communication. The sensor and patch function with an evaluation device 3, which comprises a computing device 31, a display 32 and a second communication element (33) for wireless communication, together to form a kit for determining a blood sugar level. In a method which uses said kit, the sensor 1 is implanted under a patients skin and the electrical patch 2 is positioned on the patients skin. Then properties of the glucose-binding protein 12 are measured using the measuring element 21. The measurement result is transmitted wirelessly via the first communication element 22 to a second communication element 33 of an evaluation device 3, and the patients blood sugar level is calculated from the measurement result and a reference value.
