Amyloid stains positively with congo red, periodic acid–Schiff (PAS), crystal violet, and toluidine blue. Characteristically, the congo red stain produces an apple-green birefringence under polarised microscopy.
Differential diagnosis for systemic amyloidosis
Other conditions that should be considered include:
A form of primarycutaneous amyloidosis — the most important differential. Keratinimmunohistochemistry is usually positive in lichen amyloidosis and macular amyloidosis. A primary vascular location of deposition points to systemic amyloidosis
Erythropoietic protoporphyria — hyaline deposits are seen around and within superficial blood vessel walls. Involvement of the dermoepidermal junction may be seen
Lipoid proteinosis — dermal deposits are also accentuated around blood vessels, however the material is usually weakly positive or negative for congo red. Clinical correlation can be very helpful.