SFTPD and idiopathic pulmonary fibrosis: In a large cohort of IPF patients registered in Japan’s medical subsidy system for intractable diseases, 89.3% and 92.6% of all patients—and even 84.7% and 85.9% of those with the mildest disease (Stage I of the Japanese severity classification [19])—had serum SP-D and KL-6 levels exceeding the respective cutoff values of 110 ng/mL and 500 IU/mL [12].