SFTPD and idiopathic pulmonary fibrosis: To test this hypothesis, we quantitated levels of circulating napsin A in patients with IPF, primary pulmonary adenocarcinomas, and controls, after that we analyzed the correlations between the serum levels of napsin A and those of KL-6, SP-A, SP-D respectively, and lung function as measured by percent-predicted forced vital capacity (FVC) in IPF patients.