How is amyloidosis diagnosed

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 pad57%–85%
Bone marrow30%–50%
Rectal mucosa50%–84%
Gingiva/labial salivary gland60%
Skin50%

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:

Kidney90%–98%
Carpal ligament90%–95%
Liver92%–96%
Sural nerve100%
Skin45%–83%
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