Background: We have previously reported sputum YKL-40 could be a biomarker for predicting neutrophilic airway inflammation in asthma, COPD, and asthma-COPD overlap (ACO) (Suzuki Y. ERS 2019). However, little is known about relationship between serum YKL-40 and not only airway inflammation but also clinical features and future risks in obstructive airway disorders.

Objectives: To evaluate associations between serum YKL-40 levels and clinical parameters related to disease activity such as lung function among patients with asthma, COPD, and ACO.

Methods: Forty-three asthmatics (BA), 27 COPD patients, and 31 ACO patients were recruited and followed-up for more than 2 years. Blood samplings, induced sputum tests, and spirometry were performed at entry. Annual decline of FEV₁ was also evaluated.

Results: There were significantly negative correlations between serum YKL-40 levels and annual decline of FEV₁ as well as FEV₁/FVC and FEV₁(% predicted) (p<0.05). Additionally, in 15 patients (5 BA, 3 COPD, and 7 ACO) whose serum YKL-40 and FEV₁ were measured simultaneously twice within 6 months, changes of serum YKL-40 levels negatively correlated with those of FEV₁ (p<0.05). Moreover, serum YKL-40 levels showed a significant positive correlation with sputum neutrophil percentage (p<0.01). Finally, serum YKL-40 levels in the COPD and ACO group, where neutrophilic airway inflammation are predominant, were significantly higher than those in the BA group (p<0.05).

Conclusions: Serum YKL-40 can be a biomarker of neutrophilic airway inflammation and provide useful information on predicting progression of airflow limitation among patients with asthma, COPD and ACO.