Of note, several protein biomarkers measured in the blood are already known: For instance, fibrinogen and C-reactive protein are correlated with COPD severity and risk of exacerbations [133,134]; soluble receptor for advanced glycation end-products (sRAGE) is inversely correlated with emphysema and airflow obstruction [135]; surfactant protein D (SP-D) has been associated with COPD and emphysema [136,137]; and club cell-16 (CC16) might be correlated with airflow obstruction and emphysema [137,138]. Here, SFTPD is linked to pulmonary emphysema.