Redstaining

Redstaining refers to the application of red or reddish dyes to a sample in order to impart a red coloration for visualization, identification, or quantification. It is a common component of histology, microbiology, and materials science, where color differences facilitate interpretation of structure, composition, or pathology. Red stains can act as counterstains or as primary stains, depending on the protocol and the target tissue or organism. Examples include eosin, which provides a pink-to-red counterstain in hematoxylin–eosin preparations, and Congo red, used to detect amyloid deposits.

In microbiology and clinical pathology, red staining often reveals specific features: safranin is used as a red counterstain in Gram staining to color Gram-negative bacteria, while carbol fuchsin yields red acid-fast organisms in Ziehl–Neelsen-type procedures. Congo red-stained amyloid appears red-orange and may exhibit apple-green birefringence under polarized light, aiding confirmation. The choice of red dye and the accompanying counterstain determine which structures appear red and how contrast is achieved.

Procedure and interpretation typically involve fixation, sectioning or smearing, staining, and rinsing, followed by mounting and examination under light or polarized light microscopy. Red staining is influenced by factors such as pH, solvent, and staining time; improper differentiation can obscure results. Limitations include nonspecific staining, color overlap with other tissue components, and artifacts from endogenous pigments or overlapping colors. Controls and standardized protocols are essential for reliable results.