C4A and systemic lupus erythematosus: In regard to RDW, it is a readily available parameter that have shown that could be an interesting tool to assess SLE disease activity since it has been associated with C3, C4, CRP, ESR, IgM, SLEDAI-2K score [[6], [7], [8], [9], [10], [11]] or even to certain symptoms such as fatigue [12].