Abstract: Devices, methods, and kits for measuring or otherwise evaluating the concentration of one or more analytes in a body fluid are described. The devices, methods, and/or kits may be non-invasive. In some variations, a method for measuring the concentration of an analyte in sweat of a subject may comprise contacting a colorimetric membrane with a skin surface of the subject so that the membrane collects a volume of sweat from the skin surface, and analyzing the colorimetric membrane to determine the concentration of the analyte in the collected volume of sweat.