Which stain is used on pre-treatment slides to differentiate AL from AA amyloid?

• A. Congo Red
• B. PAS
• C. Schiff's reagent
• D. Hematoxylin

Answer: (A)

Differentiating AL versus AA amyloid histochemically relies on how the amyloid responds to oxidative pretreatment. Congo Red is the stain used to visualize amyloid; it binds to the beta-pleated sheet structure and, when viewed under polarized light, produces an apple-green birefringence that confirms amyloid presence.

To tell the types apart, the tissue is pretreated with potassium permanganate. AA amyloid is sensitive to this oxidation and loses Congo Red binding after treatment, so the Congo Red staining (and its birefringence) disappear. AL amyloid, on the other hand, is resistant to KMnO4 oxidation and retains Congo Red staining after pretreatment, so birefringence can still be observed.

Thus, Congo Red on pretreated slides is used to differentiate AL from AA amyloid. Other stains listed—PAS, Schiff’s reagent, and hematoxylin—don’t provide this differential capability.