Since the presence of anti-dsDNA antibodies defines patients with ‘prototypical/classic’ SLE and serum complement C3 and/or C4 are a measure of disease activity,30 we tested the performance of S100 serum and protein levels in patient subgroups positive or negative for anti-dsDNA antibodies and in patients with SLE with normal versus pathologically low serum complement levels (online supplementary figure 2). Here, C4A is linked to systemic lupus erythematosus.