An impedance measurement system for detecting an analyte in a sample is disclosed. The system includes first, second, and third electrodes, wherein at least a portion of the third electrode is positioned between the first and second electrodes, means for generating an electromagnetic field between the first and second electrodes, means for electrically controlling the third electrode, wherein the third electrode modifies the electromagnetic field, and a processor for detecting a presence of the analyte in the sample, based at least in part on a property of the electromagnetic field.