A histopathological scoring and grading system to predict outcome for patients with AA amyloidosis

Purpose Renal involvement is associated with significant morbidity and mortality in AA amyloidosis. Extend of amyloid deposition in kidney biopsies may be predictive for clinical manifestations and outcomes. The aim of our study is to assess clinical features of patients with biopsy-proven renal AA amyloidosis and to evaluate the relationship between histopathological scoring and grading of renal amyloid deposition with clinical findings and outcomes. Methods The study included 86 patients who were diagnosed with renal AA amyloidosis. The demographic and clinical features at the time of biopsy and follow-up data were retrospectively collected. Amyloid deposition in glomeruli, interstitium, vessels and tubulointerstitial findings were scored and renal amyloid prognostic score (RAPS) was assigned by adding all scores. RAPS was further divided into three grades (RAPS grade I, II, III). Results Median age was 50 (36-59) years. Familial Mediterranean fever was the leading cause. RAPS grade and interstitial inflammatory infiltration were associated with baseline eGFR and glomerular amyloid deposition was associated with proteinuria. During the follow-up period (median 50 months), 39 patients developed ESRD. Extensive (involving > 50%) glomerular amyloid deposition, baseline eGFR and proteinuria were independent risk factors for progression to end stage renal disease. Death censored renal survival was significantly lower among patients with RAPS grade III compared to those with RAPS grade I and II. Patient survival rate was not different according to RAPS grade. Conclusions Degree of renal amyloid accumulation is associated with renal function and outcome. The scoring and grading system may be predictive in clinical outcome and contribute to understanding of disease mechanism.