Serum YKL-40 level was higher in patients with severe AAV and high FFS compared to those without (p = 0.007 and p < 0.001); multivariable linear regression analysis revealed that serum YKL-40 was independently associated with BVAS, FFS, and SF-36 scores. The gene discussed is CHI3L1; the disease is anti-neutrophil cytoplasmic antibody-associated vasculitis.