Comparison of biomarkers of subclinical lung injury in obstructive sleep apnea

Summary Background Obstructive sleep apnea (OSA) has both systemic and local effects partly through the increased oxidative stress caused by intermittent hypoxia and reoxygenation. However, lung-specific biomarkers in OSA have not been fully assessed in comparison with systemic biomarkers such as C-reactive protein (CRP), although results of a recent study having a small sample size indicated KL-6 as one candidate. Methods Subjects of the present study were 197 patients suspected to have OSA. In addition to polysomnography, we also measured serum levels of KL-6, surfactant protein-D (SP-D) and CRP and pulmonary function. We examined the relationships of different biomarkers with OSA severity and pulmonary function. Results The apnea/hypopnea index (AHI) was significantly positively correlated with serum KL-6 levels even after adjustment for body mass index (BMI) and smoking ( p = 0.03), but not with SP-D and CRP. Also, a significant trend for an increase in serum KL-6 was noted in accordance with the severity of OSA even after adjustment for BMI and smoking ( β coefficient = 0.18, p = 0.02). Additionally, elevated KL-6 levels were significantly associated with restrictive lung function disturbance and gas exchange derangement after adjustment for obesity and smoking, which contrasted with CRP whose elevations were significantly associated with worsened airflow limitation and increased lung volume. Conclusions Serum KL-6 levels may reflect the degree of subclinical lung injury associated with OSA independently of obesity or smoking, unlike CRP. We consider that KL-6 can be a potential candidate as a lung-specific biomarker of OSA and might provide complementary information on systemic biomarkers in assessing OSA.