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How is amyloidosis diagnosed
The diagnosis of amyloidosis is only confirmed by tissue biopsy. The amyloid deposits appear as hyaline material on light microscopy. Congo-red stained tissue shows the characteristic apple green birefringence under polarized light. Thioflavine T staining yields an intense yellow-green fluorescence. Immunohistochemical staining of tissue can be done to characterize the amyloid fibril protein subunit type, especially amyloid AA and ATTR.
Which tissue should be biopsied?
A screening biopsy should be performed first because the sensitivity is good and complications are few. Screening sites and their yields are:
Abdominal fat pad | 57%–85% |
Bone marrow | 30%–50% |
Rectal mucosa | 50%–84% |
Gingiva/labial salivary gland | 60% |
Skin | 50% |
Abdominal fat pad aspiration is performed by injecting saline into the abdominal wall fat about 10 cm laterally to the umbilicus using a 16-gauge needle attached to a 20-mL syringe and sucking back. Fat obtained is processed for Congo red staining. It is positive in 80% to 90% of patients with AL or ATTR amyloidosis and in 60% to 70% of patients with AA amyloidosis. Note that 15% of patients with systemic AL amyloidosis will have both a negative abdominal fat pad and bone marrow biopsy.
If the screening biopsies for amyloid are negative, what should be done?
If biopsy screens are negative, biopsy of a clinically involved site may be undertaken, realizing that the risk of bleeding may be substantial. Therefore, do not biopsy a liver that is grossly enlarged. Yields for clinically involved sites are:
Kidney | 90%–98% |
Carpal ligament | 90%–95% |
Liver | 92%–96% |
Sural nerve | 100% |
Skin | 45%–83% |