Assay reading method

A region of interest (ROI) containing pixels associated with a test region is extracted from an image of an assay. In an aspect, the test region comprises a test line and/or control line and a continuous block of colour. Respective intensity values of pixels in the ROI corresponding to the block are estimated and a flatness value determined. A reflection in the image is detected if the flatness exceeds a threshold value. In an aspect, the assay is a lateral flow assay and the test region includes a test line or control line of a test strip. The ROI is divided into pixel lines, each running through the test or control line parallel to a flow direction of the test strip. A number of the pixel lines are selected randomly. For each pixel position along the lines, an average intensity value is calculated from the corresponding pixels in each of the randomly selected lines.