Which staining method is the gold standard for confirming amyloid deposition in tissue biopsies?

Explanation

## Histological Diagnosis of Amyloidosis **Key Point:** Congo red staining with apple-green birefringence under polarized light is the gold standard and pathognomonic for amyloid identification. ### Congo Red Staining Mechanism Congo red is an azo dye that binds to the β-pleated sheet structure of amyloid fibrils. When viewed under ordinary light, amyloid appears pink-red. Under polarized light microscopy, the birefringence is rotated 90° relative to the dye orientation, producing characteristic **apple-green birefringence**. ### Why This Is Pathognomonic - The apple-green birefringence is virtually diagnostic of amyloid - No other tissue component produces this specific optical property - Sensitivity and specificity are both >95% ### Other Staining Methods (Historical/Supplementary) | Stain | Appearance | Use | Limitation | |-------|-----------|-----|------------| | Thioflavin T | Blue fluorescence | Research, quantification | Not routine diagnostic | | Crystal violet | Metachromasia (pink → purple) | Older method | Less specific than Congo red | | H&E | Pink, homogeneous | Screening only | Not diagnostic without Congo red | | Immunohistochemistry | Specific antibodies | Amyloid typing (AL, AA, etc.) | Requires Congo red confirmation first | **High-Yield:** Always remember: **Congo red = apple-green birefringence = amyloid confirmed**. This is tested in nearly every pathology exam. **Mnemonic:** **CAP** — **C**ongo red, **A**pple-green, **P**olarized light = Pathognomonic for amyloid. **Clinical Pearl:** If Congo red staining is negative, amyloidosis is essentially ruled out. If positive, immunohistochemistry can then determine the amyloid type (AL, AA, etc.).