1. A method for determining the intensity of a test strip comprising: obtaining an image comprising a sample line in a test strip and a plurality of reference lines, wherein the reference lines have known intensities; determining grayscale values of the sample line and the reference lines from the image; constructing a standard curve based on the grayscale values versus the known intensities of the reference lines; and determining the intensity of the sample line by fitting the grayscale value of the sample line on the standard curve.