The present disclosure is directed to a method and apparatus for performing an assay on a dry slide or other solid media using a reduced reading window. In an embodiment, method of performing at least one assay comprises obtaining an image of a fluid sample located on a dry slide, positioning a reading window to correspond to an area of the fluid sample in the image, determining an interference area within the reading window based on light intensity, reducing the reading window to eliminate the interference area from the reading window, and performing at least one assay using the reduced reading window.