The serum YKL-40 and CRP levels of COPD patients in the exacerbation group were higher than those in stable group (YKL-40: 78.6 [52.3–122.2] ng/ml versus 46.7 [31.2–75.5] ng/ml; p = 0.0005; CRP: 60.0 [39.9–93.4] mg/L versus 16.9 [13.0–23.8] mg/L; p < 0.001) (Fig. 1b, c). Here, CHI3L1 is linked to chronic obstructive pulmonary disease.