Satoskar AA et al. (2007) Typing of amyloidosis in renal biopsies: diagnostic pitfalls. Arch Pathol Lab Med 131: 917–922

The most common nonfamilial forms of amyloidosis that involve the kidney are the AL (amyloid light chain) and AA (serum amyloid A) types. These two forms require different treatment strategies, and reliable differentiation between types is, therefore, clinically important. Satoskar and co-workers have analyzed 15 renal biopsy samples and used their results to illustrate some of the potential problems in classification of renal amyloidosis.

AL amyloidosis is characterized by deposits that contain monoclonal immunoglobulin light chains; either the κ or the λ light chain is expected to predominate. The first step in differentiating AA from AL amyloidosis is, therefore, immunofluorescence staining for these two proteins. There were 13 cases of AL amyloidosis in this study; biopsy samples from 11 of these individuals exhibited predominance of either κ or λ light chains. Four samples showed only moderate—rather than the expected intense—staining of the predominant light chain. All 13 samples were, however, negative when stained for AA. Renal tissue from both cases of AA amyloidosis exhibited strong positive immunostaining for AA but, unexpectedly, light chains were also detected.

The authors conclude that relying solely on immunostaining of light chains could lead to erroneous diagnosis of renal amyloidosis subtype. They recommend staining for AA protein in all cases, except those in which the light chain that is predominant in the amyloid deposits is also detected in both serum and urine. If doubt remains, consultation should be obtained from a center capable of performing more-extensive and more-detailed immunostaining for the other amyloidogenic proteins.