Congo red and Thioflavin T prefer Orth fixation. True or False?

• A. True
• B. False
• C. Not sure
• D. Both fixation methods are equivalent

Answer: (B)

Amyloid stains rely on preserving the beta-pleated sheet structure of the proteins so Congo red can bind and show apple-green birefringence under polarized light, and Thioflavin T can fluoresce when bound to the fibrils. The fixative used can dramatically affect that binding and the fluorescence. Orth's fixative is mercury-based and can modify tissue proteins or introduce metals that interfere with dye binding and quench fluorescence. Formalin-based fixation, especially neutral buffered formalin, tends to preserve the amyloid structure and allows both Congo red binding and Thioflavin T fluorescence to be observed reliably on paraffin sections. Because of these effects, they do not prefer Orth fixation.