A system and method is provided for detecting concentration of an analyte in a fluid. The method comprises detecting an optical property of a first region of two or more regions in a system, the first region located in a container having a reservoir for one or more modifiers of one or more optical properties of the first region. The movement of the one or more modifiers is responsive to changes in concentration of the analyte. A next step detects an optical property of a second region of the two or more regions in the system, the second region located in a container having a reservoir for one or more modifiers of one or more optical properties of the second region. The movement of the one or more modifiers is responsive to changes in concentration of a compound, where the compound is something other than the analyte. The detected optical property of the first region and the detected optical property of the second region are used in embodiments to separate the effect of the analyte on the detected optical property of the first region from the effect of the compound, where the compound is an interfering compound.