The Congo red stain can be used to identify amyloid. The recognition of amyloid requires properly prepared slides and careful interpretation.
Causes of weak staining:
(1) prolonged fixation in a formaldehyde-based fixative
(2) unprocessed cut sections stored for some time (weeks or months)
(3) deterioration of Congo red stain
Causes of nonspecific staining:
(1) pH too low
(2) connective tissue fibers (loses dichromism when rotated under polarized light)
(3) sodium concentration too low
Causes of incorrect color under polarized light:
(1) section too thick or thin (should be 6-8 microns).
It is important that the positive Congo red control should have the same thickness as the patient sample and be recently cut.
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Specialty: Clinical Laboratory
