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Complement activation plays a key role in the pathogenesis of lupus nephritis (LN), a severe complication of systemic lupus erythematosus (SLE). We prospectively evaluated 15 LN subjects and two control groups: 13 non-SLE renal subjects (control A) and 239 SLE subjects without LN (control B). All had C4d levels on circulating erythrocytes (E-C4d), reticulocytes (R-C4d) and platelets (P-C4d) measured by flow cytometry, while C4d deposition in renal tissue was semiquantitatively assessed in LN subjects and control A using immunoperoxidase staining.Compared with control A, LN biopsies had higher glomerular-C4d scores (p = 0.003), which were associated with more frequent granular glomerular immunofluorescence staining and electron dense deposits (p < 0.001). Compared with control A and B groups, LN subjects had higher E-C4d (p = 0.002 and p = 0.005) and R-C4d levels (p = 0.002 and p = 0.008), respectively. LN subjects were more likely to have P-C4d compared with control A (p = 0.016). In LN, only E-C4d correlated with National Institutes of Health (NIH) activity index (r = 0.55, p = 0.04).In conclusion, LN biopsies showed frequent glomerular-C4d staining associated with immune complex deposits. LN subjects had higher E-C4d and R-C4d levels compared with both control groups. E-C4d levels also correlated with NIH activity index. These findings suggest a potential role of C4d on circulating cells as a biomarker for lupus nephritis. [ABSTRACT FROM AUTHOR]