Clinical usefulness of serum KL-6 and SP-A levels as biomarkers of pulmonary fibrosis in healthy subjects

<bold>Background:</bold> Interstitial lung abnormalities (ILAs) are associated with the risk of progression to interstitial lung diseases (ILDs). Krebs von den Lungen 6 (KL-6) and surfactant protein (SP)-A have been used as biomarkers of ILDs. In this study, we evaluated the levels of these biomarkers and identified their clinical correlations in healthy individuals to assess their usefulness in the diagnosis of ILAs. <bold>Methods:</bold> The patient samples were categorized into three groups: healthy, disease, and ILD groups. We used the automated immunoassay KL-6 and SP-A assay kits. The analytical performance evaluation involved precision, linearity, comparison, establishment of reference intervals, and cutoff points. We also analyzed the correlations between the presence of abnormalities on chest radiography and computed tomography (CT) or pulmonary function test (PFT) and serum levels in the healthy group. <bold>Results:</bold> KL-6 and SP-A assays showed good analytical performance. The KL-6 and SP-A cutoff values were 304 U/mL and 43.5 ng/mL between the ILD and healthy groups, respectively, which were lower than the values recommended by the manufacturer. In the clinical correlations with radiological findings, SP-A values in subjects with lung abnormalities on CT scans were significantly higher than those in normal scans. There was no significant difference in KL-6 and SP-A levels among PFT patterns; however, both serum levels in the mixed pattern showed higher values than those in the other patterns. <bold>Conclusions:</bold> The results revealed a positive association between increased levels of SP-A and KL-6 and clinical characteristics in ILAs.