Abstract: A method for determining the concentration of an analyte in a sample is disclosed which comprises contacting the sample with a micro electrode which comprises an enzyme capable of reacting with said analyte and a redox mediator which is capable of being converted by being oxidised or reduced by said enzyme once the latter has reacted with the analyte, allowing the analyte to react with the enzyme, then applying a potential across the electrode and measuring the resulting concentration of the converted mediator electrochemically.