SFTPD and interstitial lung disease: The CTD-ILD group exhibited significantly higher levels of C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and all four serum biomarkers (KL-6, SP-A, SP-D, and SF) (all <i>P</i> < 0.05), alongside impaired pulmonary function and a higher CPI (<i>P</i> < 0.05).