Background: Surfactant protein D (SP-D) is a promising blood biomarker in patients with chronic obstructive pulmonary disease (COPD). Nevertheless, circulating levels of SP-D are not related to pulmonary functions. In the present exploratory study, we created a simple index of plasma to bronchoalveolar lavage (BAL) fluid ratio of SP-D (pSP-D/bSP-D), and determined whether this index would relate to the severity of airflow limitation and hence represent a superior biomarker than pSP-D alone.
Material/Methods: In 50 ex and current smokers (mean age 57.6±7.8 years, 74% men), SP-D was measured in BAL fluid and plasma samples, and the relationships between spirometric variables and a composite parameter – the pSP-D/bSP-D ratio were determined.
Results: There was a significant inverse correlation between the pSP-D/bSP-D ratio and the severity of airflow obstruction, as measured by FEV1/FVC (p=0.012). In contrast, no relationship was observed between FEV1/FVC and pSP-D alone.
Conclusions: We suggest that integrating both lung and plasma expression of pneumo-proteins may be more useful than plasma expression alone in developing pneumo-proteins as potential biomarkers in COPD.

Keywords: Pulmonary Surfactant-Associated Protein D - blood, Pulmonary Disease, Chronic Obstructive - pathology, Lung - metabolism, Cross-Sectional Studies, Bronchoalveolar Lavage Fluid - chemistry, Biological Markers - blood, Respiratory Function Tests, Smoking - physiopathology