Examples of noncellular debris screened by filters (a, c, e, g: Hoechst images; b, d, f, h: cytokeratin images). a, b Large area of nonspecific staining containing nuclei. Standard deviation refers to the staining intensity of Hoechst and cytokeratin within an outline. The cytokeratin standard deviation is 26.18%, and the Hoechst standard deviation is 26.35%. This feature is classified as debris since the standard deviations of the 2 channels are within 5%. c, d Piece of dust fluorescing similarly in both channels. With a cytokeratin and Hoechst correlation coefficient of 0.896, the piece of dust is determined to be noncellular. e, f Fiber with highly uniform fluorescence. The Hoechst standard deviation is 10.36%, exhibiting hardly any variability across the feature. g, h Cells near the edge of a large air bubble generate an outline with a large amount of empty space. A circularity of 0.06 indicates that the outline is erroneous.