What is the role of biopsy in the workup of AA (inflammatory) amyloidosis?

Updated: Aug 11, 2020
  • Author: Richa Dhawan, MD, CCD; Chief Editor: Herbert S Diamond, MD  more...
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Answer

For the detection of amyloid, biopsy of a clinically affected organ is the most sensitive method and may also detect concomitant pathologies. However, such a biopsy is invasive and carries the risk of complications, in particular bleeding. Thus, if amyloidosis is clinically suspected, a less invasive procedure may be desirable.

In the early 1970s, Westermark and Stenkvist demonstrated that amyloid can be detected in subcutaneous fat. During the decades, subcutaneous fat pad biopsy, obtained via fine-needle aspiration, being safe, cheap, and rapid, has been introduced as a screening test for the detection of amyloidosis. [31]   Nevertheless, the tissue with the highest yield, particularly in the presence of proteinuria or renal failure, is the kidney. Technically adequate samples have a diagnostic yield close to 100%.

Rectal biopsy is more useful than subcutaneous fat aspiration in AA amyloidosis. It has been found to produce positive results (assuming that submucosa is included in the biopsy specimen) in 80-85% of patients ultimately found to have tissue amyloid at a clinically relevant site. Samples from either the subcutaneous fat aspirate or the rectal biopsy can be stained as conventional tissue biopsies to determine the presence and nature of the amyloid precursor.

Occasionally, patients have positive results on subcutaneous fat aspirates in the presence of a negative result on rectal biopsy, while others may have deposits in the rectal tissue and not in the aspirate. Use of both procedures may increase the yield to 90%. Abdominal subcutaneous fat biopsy results are not very sensitive in AA amyloidosis caused by FMF and in dialysis-related amyloidosis. The results are usually negative, probably because beta2-microglobulin does not accumulate in this tissue.

Series from individual centers have shown that the labial gland or gastric mucosal biopsies can also be high-yield procedures, but these have not been used widely for amyloidosis, and their general utility remains to be definitively established.

Congo red staining

Congo red stain continues to be the criterion standard for detection of amyloid deposits. In AL and AA amyloidosis, Congo red staining of aspirated subcutaneous abdominal fat has a sensitivity of 70-90% for the diagnosis. Kidney, heart, or liver samples have also been used for Congo red staining, but biopsy of rectal mucosa, skin, or subcutaneous fat is often sufficient, except in the cases of FMF, when it is rarely, if ever, positive.

The tissue is stained with an alkaline solution of Congo red, and examined it under polarized light, where positive (green) birefringence is detectable in the presence of amyloidosis of any type. The nature of the fibril precursor can be established by immunohistochemical staining with antibodies specific for the major amyloid precursors (AA, immunoglobulin L chains of κ or λ type, antitransthyretin). In AA amyloidosis, only the AA is positive. The amyloid nature of the deposit can by confirmed by staining with an antiserum specific for serum amyloid P-component (SAP).

Once histological diagnosis of amyloidosis has been established, the amyloid type should be defined based on immunohistochemical analysis and genetic testing. Immunoelectron microscopy characterizes the amyloid deposits by co-localizing the specific proteins with the fibrils and can be performed on abdominal fat samples.


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