A method for detecting concentration of an analyte in a fluid, the method comprising: detecting an optical property of a first optical cavity detection region (248, 1248) of two or more optical cavity detection regions in a system, the first optical cavity detection region located in a first container (20, 220, 1220, 220a-d, 922, 924, 926); detecting an optical property of a second optical cavity detection region (250,1250) of the two or more optical cavity detection regions in the system, the second optical cavity detection region located in a second container (22, 222, 1222, 922, 924, 926); using the optical property detected from the first optical cavity detection region (248, 1248) and the optical property detected from the second optical cavity detection region (250, 1250) in determining the concentration of the analyte in said fluid; characterized in that the first container (20, 22, 1220, 220a-d, 922, 924, 926) comprises one or more modifiers of one or more optical properties of said first optical cavity detection region (248, 1248) and a reservoir (240, 244, 502, 602, 702, 802, 804, 1240, 1244) for said one or more modifiers, said reservoir (240, 244, 502, 602, 702, 802, 804, 1240, 1244) being separate from said first optical cavity detection region (248, 1248) and comprising a receptor that binds reversibly with said one or more modifiers, said receptor being constrained to said reservoir (240, 244, 502, 602, 702, 802, 804, 1240, 1244), the movement of said one or more modifiers between said reservoir (240, 244, 502, 602, 702, 802, 804, 1240, 1244) and said first optical cavity detection region (248, 1248) being responsive to changes in concentration of said analyte. detecting an optical property of a first optical cavity detection region (248, 1248) of two or more optical cavity detection regions in a system, the first optical cavity detection region located in a first container (20, 220, 1220, 220a-d, 922, 924, 926); detecting an optical property of a second optical cavity detection region (250,1250) of the two or more optical cavity detection regions in the system, the second optical cavity detection region located in a second container (22, 222, 1222, 922, 924, 926); using the optical property detected from the first optical cavity detection region (248, 1248) and the optical property detected from the second optical cavity detection region (250, 1250) in determining the concentration of the analyte in said fluid; characterized in that the first container (20, 22, 1220, 220a-d, 922, 924, 926) comprises one or more modifiers of one or more optical properties of said first optical cavity detection region (248, 1248) and a reservoir (240, 244, 502, 602, 702, 802, 804, 1240, 1244) for said one or more modifiers, said reservoir (240, 244, 502, 602, 702, 802, 804, 1240, 1244) being separate from said first optical cavity detection region (248, 1248) and comprising a receptor that binds reversibly with said one or more modifiers, said receptor being constrained to said reservoir (240, 244, 502, 602, 702, 802, 804, 1240, 1244), the movement of said one or more modifiers between said reservoir (240, 244, 502, 602, 702, 802, 804, 1240, 1244) and said first optical cavity detection region (248, 1248) being responsive to changes in concentration of said analyte.