A filter system includes a housing, a filter medium disposed within the housing, an optical analyte sensor, and an optical reader. The optical analyte sensor is characterized by a first region that exhibit a first response to an analyte of interest and a second region that exhibit a second, different, response to an analyte of interest. The optical analyte sensor includes a detection medium and is disposed within the housing such that the detection medium is in fluid communication with the filter medium. The optical reader includes at least one light source and at least one detector.