C4A and systemic lupus erythematosus: A subset of the samples from patients with SLE used in the heterotrimer assay had previously been assessed for various markers of disease activity, including patient SLE Disease Activity Index (SLEDAI) scores at the time of serum draw, erythrocyte sedimentation rate (ESR), concentrations of anti-double stranded DNA (dsDNA) antibodies, and levels of the complement components C3 and C4.