For 34 patients with active AAV, the serum concentration of S100A8/A9 was positively correlated with the level of MPO-ANCA, serum ferritin, C-reaction protein (CRP), D-dimer, erythrocyte sedimentation rate (ESR), and rheumatoid factor (RF) (Fig. 2A–F), negatively correlated with serum albumin (Fig. 2G), and had no correlation with Birmingham vasculitis activity score (BVAS) (r = 0.215, p = 0.229). Here, MPO is linked to vasculitis.