Nonamyloid green birefringence following Congo red staining

Area of Science:

• Neuropathology
• Histopathology
• Diagnostic Techniques

Background:

• Congo red staining is a standard method for detecting amyloid deposits in tissue biopsies.
• Green birefringence under polarized light is characteristic of amyloid but can be mimicked by other substances.
• Electron microscopy is typically used to confirm amyloid presence at the ultrastructural level.

Purpose of the Study:

• To investigate the cause of false-positive Congo red birefringence in muscle and sural nerve biopsies.
• To determine the influence of staining techniques, dyes, and fixatives on birefringence results.
• To establish reliable methods for differentiating true amyloid from artifactual birefringence.

Main Methods:

• Evaluation of muscle and sural nerve biopsy specimens exhibiting Congo red birefringence without electron microscopy confirmation of amyloid.
• Systematic testing of three different dyes, two staining procedures, and six fixatives.
• Analysis of the relationship between dye retention, fixative type, and observed birefringence.

Main Results:

• False-positive green birefringence was observed in Congo red-stained sections.
• The staining technique was identified as the most critical factor influencing birefringence.
• Excessive retention of Congo red dye in the tissue was strongly correlated with artifactual birefringence, particularly when using fixatives other than Carnoy's mixture.

Conclusions:

• Artifactual birefringence mimicking amyloid can occur in neuropathology specimens.
• Careful control of staining procedures and dye concentration is essential to avoid false positives.
• Accurate interpretation requires considering technical factors alongside morphological findings.