C3 and systemic lupus erythematosus: CRP: C-reactive protein, GFR: glomerular filtration rate, C3: complement C3, C4: complement C4, ANA: antinuclear antibodies, anti-ds-DNA Ab: antibodies against double-stranded deoxyribonucleic acid, proteinuria g/24h: proteins from 24 hours collected urine, SLEDAI/r: Systemic Lupus Erythematosus Activity Index/renal, LN: lupus nephritis