Fig. 1

Serum YKL-40 levels in patients with COPD and controls. Serum YKL-40 was increased in smokers with COPD (defined as COPD (+)) compared with smokers without COPD (defined as COPD (−)) and non-smokers (a); When patients were stratified according to exacerbation attacks, the serum YKL-40 and C-reactive protein (CRP) levels in patients in the exacerbation group were higher than those in the stable group (b, c); Spearman rank correlation coefficient showed that YKL-40 was positively associated with CRP (d); Borderline significance was evident between YKL-40 concentrations and pack-years (r = 0.224, p = 0.045) (e); Patients in the exacerbation group, after acute exacerbations, demonstrated decreased serum YKL-40 levels compared with those during AECOPD (f); Receiver operating characteristic (ROC) curve for distinguishing patients with AECOPD from those with stable COPD. The area under the ROC curve was 0.71 (95%CI: 0.596 to 0.823). Horizontal bars represent median values (g)