TNFSF13B and systemic lupus erythematosus: This correlation in the subgroup of patients with nephritis may be due to parallel changes in the levels of both cytokines, but also to the fact that the levels of heterotrimers are higher in this subgroup (because both BAFF and APRIL ELISA assays detect heterotrimers) Previous studies reported higher levels of heterotrimers in patients with SLE compared with healthy control and patients with rheumatoid arthritis7,8.