Prevalence and Origin of Amyloid in Kidney Biopsies

We aimed to reassess renal amyloidosis in kidney biopsies with a focus on possibly misclassified or unclassified cases and changes in the prevalence of different amyloid types. Two hundred thirty-three kidney biopsies obtained from 231 patients diagnosed with amyloid during the period from 1990 to 2007 years were included in this retrospective study. Amyloid was identified by Congo red staining and polarization microscopy. Immunohistochemical classification was made with antibodies directed against AA amyloid, apolipoprotein A1, fibrinogen, lysozyme, lambda-light chain, kappa-light chain, beta 2-microglobulin, transthyretin, and amyloid P-component. Amyloid was present in each biopsy as vascular, tubulo-interstitial and/or glomerular deposits. Immunoglobulin light chain-derived (AL) amyloidosis was most prevalent and diagnosed in 123 (53.2%) patients. It was categorized into AL amyloid of lambda-light chain (AL lambda) [105 (85.4%) patients] and kappa-light chain origin (AL kappa) [10 (8.1%)]. The amyloid deposits of 8 (6.5%) patients were not clearly distinguishable into AL lambda amyloid or AL kappa amyloid and categorized as AL amyloid, not otherwise specified. Reactive systemic amyloid A (AA) amyloidosis was the second most common type and was found in 93 patients (40.3%). Overall 7 patients were found to suffer from fibrinogen A alpha-chain-[amyloid of fibrinogen (AFib); 4 (1.7%) patients], transthyretin-[amyloid of transthyretin (ATTR); 2 (0.9%)], or apolipoprotem A1-derived (AApoAI) amyloidosis [1 (0.4%)]. In 8 patients (3.4%) the amyloid deposits remained unclassifiable. After additional immunostaining and further clinical information the diagnoses of 12 patients (5.1%) were modified (2 AL lambda amyloid, 4 AL kappa amyloid, I amyloid unclassified, 3 mixed-type amyloidosis, AA + ATTR, AL lambda + ATTR, and AL kappa + ATTR, 1 AFib, and 1 AApoAI). Although the histologic and immunohistochemical reevaluation confirmed the classifications in 221 (95.7%) patients. Renal amyloidosis is most commonly of AL lambda-origin, followed by AA amyloidosis. AFib amyloidosis was found to be the most prevalent type of hereditary renal amyloidosis, illustrating the necessity of a thorough classification of the amyloid proteins.