Stains & CD markers
Congo red

Last author update: 28 June 2022
Last staff update: 28 June 2022

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PubMed Search: Congo red

Mariel Molina Nunez, M.D.
Kenneth A. Iczkowski, M.D.
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Cite this page: Molina Nunez M, Iczkowski KA. Congo red. website. Accessed April 14th, 2024.
Definition / general
  • Congo red is a direct diazo dye used for staining amyloid in tissue sections (Biosci Rep 2019;39:BSR20181415)
  • This organic compound forms complexes with the misfolded proteins deposited in amyloidosis; analysis of the emission spectra from these complexes reveals the optical reactivity under polarized light, described as apple green birefringence (Acta Biochim Pol 2019;66:39)
Essential features
  • Also called amyloid stain
  • Amyloidosis is a rare and heterogeneous group of disorders that is characterized by the deposition of abnormally folded proteins (Acta Haematol 2020;143:322)
  • A key event in the development of AL amyloidosis is the change in the secondary or tertiary structure of an abnormal monoclonal light chain, which results in an unstable conformation
  • This conformational change is responsible for abnormal folding of the light chain, rich in β leaves, which assemble into monomers that stack together to form amyloid fibrils (Orphanet J Rare Dis 2012;7:54)
  • Amyloid formation involves a combination of several factors, including a prolonged increase in concentration of proteins that are prone to misfolding as a result of an acquired or hereditary mutation or wild type proteins with an intrinsic propensity to misfold, or a proteolytic remodeling of a wild type protein into an amyloidogenic fragment
    • Under normal conditions, misfolded proteins are promptly eliminated by the protein quality control systems; amyloidosis occurs when these systems are overwhelmed by an increased supply of misfolded proteins or when their processing capacity is reduced due to aging (Acta Haematol 2020;143:322)
  • Amyloid fibril Congo red complex demonstrates green birefringence owing to the parallel alignment of dye molecules along the β pleated sheet
Clinical features
  • Per the International Society of Amyloidosis, these disorders are classified as systemic or localized and as acquired or hereditary based on the pathogenesis (Amyloid 2020;27:217, J Intern Med 2021;289:268)
  • Most common types are AL, AA, ATTR (amyloid transport protein transthyretin) and dialysis related amyloidosis or beta2M type (StatPearls: Amyloidosis [Accessed 19 April 2022])
  • Congo red stain is mostly used for systemic or localized amyloidosis, such as AL amyloid seen in clonal proliferation plasma cell dyscrasias, AA amyloid associated with inflammatory conditions, Aβ amyloid fibrils in Alzheimer disease and TTR amyloidosis due to either familial gene mutation or wild type protein (formerly called senile amyloidosis)
  • Clinical presentation is heterogeneous and the severity depends on the organ affected; most patients with AL suffer from monoclonal gammopathy of undetermined significance (Nephrol Dial Transplant 2019;34:1460)
  • Cardiac involvement is most often diagnosed in patients with light chain and wild type or hereditary ATTR amyloidosis (Clin Med (Lond) 2018;18:s30)
  • Amyloids can be detected in the kidney, liver, heart, central nervous system, peripheral nervous system, autonomic nervous system, skin and other organs
  • Diagnosis relies on the identification of the fibrillar deposits in tissues and typing of the amyloid (Kasper: Harrison's Manual of Medicine, 20th Edition, 2019)
  • While biopsy on a target organ is most sensitive if amyloid is clinically suspected, a less invasive biopsy is recommended (e.g., abdominal fat) (Hum Pathol 2018;72:71)
  • Emanation of new techniques of proteomic analysis such as mass spectrometry / laser microdissection, has provided greater accuracy in amyloid typing (Protoplasma 2020;257:1259)
  • Amyloid deposits in tissue exhibit a deep red or salmon color, whereas elastic tissue remains pale pink
  • When viewed under polarized light, amyloid deposits exhibit apple green birefringence, orange-red fluorescence using Texas red filter visualized under ultraviolet light
  • Thickness of the section is critical (8 - 10 μm) (Gattuso: Differential Diagnosis in Surgical Pathology, 2nd Edition, 2010)
  • Rotating the slide or the polarizing filter is important for visualizing the birefringence (Diagn Pathol 2019;14:57)
  • Improved sensitivity is reported when using a metallurgical polarized microscope (Diagn Pathol 2019;14:57)
  • Tissue elements such as collagen, elastin (and others) may display birefringence of varying colors that may be misinterpreted as amyloid (Hum Pathol 2014;45:1766)
    • Do not mistake the blue-green of collagen for the lime green of amyloid
Uses by pathologists
Prognostic factors
Microscopic (histologic) images

Contributed by Christian M. Schürch, M.D., Ph.D. and Kenneth A. Iczkowski, M.D.


Transthyretin amyloidosis

Distinguishing the correct color

Rotation to visualize Congo red

Optimized AL amyloid imaging

Breast amyloidosis

Positive staining - normal
  • None
Positive staining - disease
Negative staining
Sample pathology report
  • Heart, endomyocardium, biopsies:
    • Polarizable material evident by Congo red stain, consistent with amyloid protein
Board review style question #1

Which of the following lesions should be negative for Congo red?

  1. Bone marrow in a patient with multiple myeloma
  2. Hyalinizing trabecular adenoma of the thyroid gland
  3. Kidney in a patient with systemic lupus erythematosus
  4. Liver in a patient with systemic light chain amyloidosis
  5. Medullary thyroid carcinoma
Board review style answer #1
B. Hyalinizing trabecular adenoma of the thyroid gland. Typical features of hyalinizing trabecular adenoma of the thyroid gland include oval and elongated nuclei, perinucleolar vacuoles, acidophilic nuclear inclusions, fine nuclear grooving and infrequent mitotic figures. Perivascular hyaline fibrosis and cell degeneration can mimic amyloid but these tumors are Congo red negative (Am J Surg Pathol 1987;11:583).

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