Which counterstain is used to color nuclei blue in Congo Red staining?

• A. Aluminum hematoxylin
• B. Methylene blue
• C. Eosin
• D. Nuclear fast red

Answer: (A)

The main idea is providing a clear blue contrast for nuclei when staining for amyloid with Congo red. Aluminum hematoxylin is the preferred nuclear counterstain because it gives a deep, crisp blue-black coloration of nuclei that remains stable under the alkaline conditions used during Congo red staining. This strong blue contrast helps nuclei stand out without altering or masking the Congo red–binded amyloid deposits, making the overall tissue architecture easier to interpret. Other stains either colors nuclei a different shade (like red with nuclear fast red), stains cytoplasm (eosin), or isn’t routinely used in this protocol, which is why aluminum hematoxylin is the best fit for this combination.