KL-6 as a Biomarker of Interstitial Lung Disease Development in Patients with Sjögren Syndrome: A Retrospective Case-Control Study.

Objective: Krebs von den Lungen-6 (KL-6) is expressed on regenerating type II pneumocytes and has been recognized as biomarkers in interstitial lung disease (ILD). We aim to identify the role of the serum KL-6 level in patients with newly diagnosed Sjögren syndrome (SS), as well as the correlation between the immunoassays. Methods: Patients with newly diagnosed SS and receiving HRCT for clinical reason during follow-up were included. Baseline KL-6 level was measured via enzyme-linked immunosorbent assay (ELISA) and latex particle-enhanced turbidimetric immunoassay (LETIA). Results: Of the 39 patients, 21 (53.85%) developed interstitial lung disease (ILD) by the conclusion of the follow-up period. The median time to diagnosis of ILD was 2.24 years (IQR 1.15-4.34) in the ILD group. The median serum KL-6 level, measured using ELISA, was 1232 U/mL (IQR 937-2242) and 764.5 U/mL (IQR 503.25-1035.75) in the ILD group and the non-ILD group, respectively (p = 0.001). The median LETIA for serum KL-6 was 329 U/mL (IQR 235-619) and 245 U/mL (IQR 215.25-291) in the ILD group and the non-ILD group, respectively (p = 0.074). Conclusion: Serum KL-6 levels were higher in newly diagnosed SS patients with ILD diagnosis during follow-up. Thus, the serum KL-6 level can serve as a valuable biomarker to identify hidden ILD in patients with newly diagnosed SS patients. However, the immunoassay procedure may influence the efficacy of the prediction and its clinical association.