Variations on the Membrane Staining Intensities of Positive Control Specimens. This is a histogram showing the amount of specimens in each range of intensities. Each bar contains cases that lie within a 20 unit range of intensities. Though most positive controls stain darkly (215–235), there is a significant number of cases in which staining intensity deviates markedly from the mode. Consequently, features that take into account the intensity of the positive control are used in this study.