To address whether endogenous heterotrimers are present in the sera of patients with autoimmune diseases, the recombinant heterotrimers were used as a standard to develop a bead-based immunoassay by using anti-APRIL capture mAb and fluorescence-labeled anti-BLyS detection mAb to quantify native heterotrimers in human sera. Here, TNFSF13 is linked to autoimmune disease.