Diagnostic Workup

The diagnosis of amyloidosis depends on the pathologic demonstration of typical congophilic deposits. The most common strategy is to take the biopsy from the most easily available tissue. Small amyloid deposits often occur in subcutaneous tissue of most people with AL or AA amyloidosis. For this reason, an abdominal fat aspirate is often used as an initial screen. However, a negative result does not exclude a diagnosis of AL, and other sites, such as rectal mucosa, marrow, and particularly the involved organ may need to be sampled.

The most important element of the diagnosis is typing of the amyloid deposit, as several amyloid states can have overlapping features. The presence of serum or urine monoclonal protein strongly supports AL amyloid. However, monoclonal proteins can be incidental in 3% of elderly patients.34 Therefore, there would be a small false positive rate if this were the only criterion used. Most amyloid treatment centers recommend typing of the tissue with commercial antisera for AA, TTR, and k and X light chains. Antisera are also available for other proteins such as fibrinogen, lysozyme, or apolipoprotein, but are reserved for more complex cases.

Once the diagnosis and type of amyloid have been established, the next step is to determine the extent of organ dysfunction. In AL amyloidosis, evaluation of renal function (e.g., 24-hour urine collection for immunofixation and total protein) and cardiac involvement (e.g., echocardiogram, holter monitor, serum troponin, and brain natriuretic peptide) should be undertaken even in the absence of symptoms. Studies to evaluate the underlying clone in AL amyloid should include bone marrow biopsy, studies to quantify monoclonal Ig in the serum and urine, serum-free light chains, serum p-2 microglobulin, and a skeletal survey to exclude lytic bone disease. Other evaluations, such as electromyogram and stool studies for malabsorption, are based on the nature of disease-related symptoms. Rabiolabeled serum amyloid P component (SAP) has been used to image amyloid involvement, but this test has limited availability.35 Approaches to visualize AL deposits in a noninvasive fashion are an area of active research.

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