Typing of amyloidosis in renal biopsies: diagnostic pitfalls.

CONTEXT: Amyloidosis represents a group of diseases with extracellular deposition of congophilic fibrils of similar morphology but differing chemical composition. The types commonly involving the kidney are AL (light chain amyloid) and AA (serum amyloid A). Familial amyloidosis can also affect the kidney, but we have not encountered such a case during the study period. Distinguishing between the AL and AA forms of amyloid is clinically important because of the different treatments and outcomes. The classification of amyloidosis is made by immunostaining with antibodies to kappa and lambda immunoglobulin light chains and for serum amyloid A protein.

OBJECTIVE: To draw attention to the nonspecific immunofluorescence staining patterns in renal biopsies with amyloidosis, causing potential diagnostic pitfalls.

DESIGN: Renal biopsies from 15 patients, including 13 cases of AL and 2 cases of AA amyloidosis, were studied. Immunofluorescence staining with routine antibody panel and immunoperoxidase staining for amyloid A were performed.

RESULTS: Of the 13 cases of AL amyloidosis, 2 cases showed little difference in staining intensity between kappa and lambda light chains (2+ and 3+, respectively) and 4 cases showed only moderate intensity (2+) of the predominant light chain. The 2 cases diagnosed as AA amyloidosis also exhibited staining for light chains. One case had strong (3+) signal for kappa and moderate (2+) for lambda light chain, while the other showed weaker staining.

CONCLUSIONS: Immunofluorescence staining for immunoglobin light chains on renal biopsy, as the first step to differentiate between AL and AA amyloidosis, may sometimes be inconclusive or even misleading. Applying amyloid A immunostain on a routine basis and detailed clinical history are essential to avoid misclassification.