CHI3L1 and chronic obstructive pulmonary disease: The serum YKL-40 levels for patients in smokers with COPD were higher than those in smokers without COPD (median [interquartile range], 60.80 [34.7–90.1] ng/ml versus 22.7 [15.6–36.4] ng/ml; p < 0.001) and those in never-smoker individuals (78.60 [52.3–90.11] ng/ml versus 20.9 [15.2–26.3] ng/ml; p < 0.001) (Fig. 1a).