Complement degradation product C3d in urine: Marker of lupus nephritis

Objective. To examine whether serum and urine C3d, a degradation product of C3, correlate with renal and extrarenal lupus activity. Methods. Serum and urinary C3d levels were measured by ELISA in 15 healthy individuals and 24 patients with systemic lupus erythematosus (SLE) (8 with inactive disease, 7 with active but nonrenal disease, 9 with active lupus nephritis). Disease activity variables like serum C3, C4, and anti-dsDNA an tibodies were also measured. Results. The median serum C3d levels were significantly higher (p < 0.01) i n patients with active (26 arbitrary units/ml; AU/ml) and inactive SLE (27 AU/ml) compared to healthy controls (11.25 AU/ml); levels were comparable i n patients with active renal and extrarenal SLE. On the other hand, urine C 3d was elevated only in patients with active SLE; its level was highest in patients with active lupus nephritis (0.87 AU/ml) compared to patients with active extrarenal diseases (0.31 AU/ml; p < 0.05), to patients with inacti ve lupus nephritis (0.06 AU/ml; p < 0.001), or to levels in healthy individ uals (0.06; p < 0.001). Urine C3d showed stronger correlation with disease activity score (SLE Disease Activity Index) than serum C3, C4, anti-dsDNA a ntibodies, and serum C3d. Conclusion. Urine C3d is a good index of active lupus, particularly lupus n ephritis.