CHI3L1 and rheumatoid arthritis: Serum YKL-40 level was higher in patients with AAV than those with SLE, RA, and OA, as well as in those of HC (median YKL-40: AAV 189.2 ng/mL vs. SLE 88.3 ng/mL vs. RA 73.7 ng/mL vs. OA 87.4 ng/mL vs. mean YKL-40: HC 99.4 ng/mL).