A sensor designed to determine the concentration of analyte in a sample having a volume of less than about 1 μL. The sensor has a working electrode coated with a redox mediator that acts as an electron transfer agent between the analyte and the electrode. A second electron transfer agent, such as an enzyme, can be added to facilitate the electrooxidation or electroreduction of the analyte. Various electrochemical detection methods, such as amperometric, voltammetric, and potentiometric techniques, can be used to determine the analyte concentration. The sensor can be used to determine the concentration of a biomolecule, such as glucose or lactate, in a biological fluid, such as blood or serum.