A method of measuring a capacitance (C) of a sensor having a working electrode that is coated by an insulating layer and a ligand forming an affinity surface. The method involves the steps of bringing the electrode into contact with an analyte, supplying a constant first current (I1), and a constant second current (I2) of opposite direction to the first current (I1), and a constant third current (I3) of same direction as the first current (I1) during determined time periods to the sensor. Further the method includes sampling the potential (V) built up across the sensor, and calculating the capacitance of the sensor from the inclination (B, D, F, H) of a potential curve received in response to the current supply. A method used for detection of an interaction between a ligand and an analyte is also disclosed.