Method of determining the concentration of an analyte in a sample of a body fluid using a camera and a color reference card

A mobile device having a camera and a processor, the mobile device configured for performing a method of determining the concentration of an analyte in a sample of a body fluid wherein the method comprises:

• i) using the camera to capture an image of at least a part of a color reference card and of at least a part of a reagent test field of an optical test strip having the sample applied thereto,
• wherein the test field is in a defined position in the image relative to the color reference card and the color reference card has a first plurality of different gray reference fields locally assigned to the test field,
• wherein the first plurality of different gray reference fields and the test field are locally assigned to each other by being placed in neighboring positions, or wherein the first plurality of different gray reference fields locally assigned to the test field are arranged on the color reference card such that the first plurality of different gray reference fields surrounds the test field,
• wherein the color reference card has a plurality of different color reference fields having known reference color values and a second plurality of different gray reference fields locally assigned to the plurality of different color reference fields, and
• wherein the second plurality of different gray reference fields and the plurality of different color reference fields are locally assigned to each other by being placed in neighboring positions or wherein the second plurality of different gray reference fields locally assigned to the plurality of different color reference fields are arranged on the color reference card such that the second plurality of different gray reference fields surrounds the plurality of different color reference fields,
• ii) applying a predetermined pixel-based mean tone map correction to the image obtained in step i), thereby obtaining a first intensity-corrected image, wherein the predetermined pixel-based mean tone map correction comprises an assignment of a second brightness value to a first brightness value, wherein the first brightness value is recorded by the camera, wherein each pixel of the recorded image is corrected individually by the predetermined pixel-based mean tone map correction;
• iii) deriving, from the first intensity-corrected image, local brightness information for at least one of the plurality of different color reference fields and for the test field by using gray reference fields of the first plurality of different gray reference fields and the second plurality of different gray reference fields which are locally assigned to the at least one color reference field and the test field, respectively, wherein the local brightness information comprises a numerical indication describing the local intensity of at least one RGB color of the at least one color reference field and the test field, respectively;
• iv) applying a mobile device-specific tone map correction to the first intensity-corrected image taking into account the local brightness information and thereby obtaining at least one second intensity-corrected image;
• and
• v) determining the analyte concentration based on a color formation reaction of the test field by using the second intensity-corrected image.