SFTPD and idiopathic pulmonary fibrosis: KL-6, SP-D, SP-A, CXCL1, CXCL9, CXCL10, and CXCL11 have been reported, but the results are not consistent, and the proposed biomarkers cannot effectively distinguish between IPF, IPAF, and CTD-ILD [4–8,31].