Methods and reagents for diagnosing, prognosing, and treating systemic lupus erythematosus (SLE) are disclosed, involving calculating an SLE risk score for a subject based on a level of each of an erythrocyte C4d (EC4d) marker, a B-cell C4d (BC4d) marker, antinuclear antibodies (ANA), anti-Smith antibodies (anti-Sm) and optional rule-out markers (SS-B/La, Scl-70, Jo-1, CENP, MCV).