SFTPD and interstitial lung disease: Beyond the elevated anti-CCP antibody and rheumatoid factor (RF), other alternative biomarkers have also been evaluated for their ability to diagnose RA-ILD, such as the epithelial cell-derived Krebs von den Lungen-6 (KL-6) (11), surfactant protein D (SP-D), matrix metalloproteinase-7 (MMP-7), interferon-γ-inducible protein 10 (IP10/CXCL10), and pulmonary and activation-regulated chemokine (PARC) (12, 13).