Use

Congo red stain is a special immunohistochemical procedure used as the gold standard to demonstrate amyloid deposits in tissue sections. It is primarily applied for the diagnosis of amyloidosis, where the amyloid‑fibril–Congo red complex displays characteristic 'apple green' birefringence under polarized light microscopy. When Congo red staining is positive, additional immunohistochemical evaluation for amyloid subtypes—such as amyloid A, amyloid P, and immunoglobulin light chains—is available if clinically indicated.