CHI3L1 and chronic obstructive pulmonary disease: Serum levels of YKL-40 in current smokers without COPD (331.8±37.0 ng/mL, n = 28) and COPD patients (268.9±32.3 ng/mL, n = 45) were significantly (P<0.01) higher than those in non-smokers (177.8±22.6 ng/mL, n = 30).