KL-6, IL-18, and S100A8/A9 are biomarkers of connective tissue disease-associated interstitial lung disease

To investigate the correlation between serum Krebs von den Lungen-6 (KL-6), Interleukin-18 (IL-18), S100A8/A9, and the diagnosis and severity of connective tissue disease-associated interstitial lung disease (CTD-ILD) by detecting the serum expression levels of KL-6, IL-18, and S100A8/A9 in patients with CTD-ILD. Serum samples of healthy controls, CTD patients without ILD, and CTD patients with ILD were collected. Enzyme-linked immunosorbent assay was used to determine serum KL-6, IL-18, and S100A8/A9 expression levels in each group. SPSS 20.0 statistical software was applied to analyze the data. Serum KL-6, IL-18, and S100A8/A9 levels in the CTD-ILD group were higher than those in the CTD and healthy controls groups, and the difference was statistically significant (P < .05). Serum KL-6 levels in the CTD-ILD group were negatively correlated with the lung function indexes (% forced vital capacity [FVC%] and % diffusing capacity of the lungs for carbon monoxide [DLCO%]) (r = -0.413, P = .017; r = -0.402, P = .023), and IL-18 and S100A8/A9 had no significant correlation with FVC% and DLCO% (P > .05). The optimal threshold value of KL-6 for diagnosis of CTD-ILD was 4.73 ng/mL (sensitivity: 89%, specificity: 62.3%, P < .05), and the optimal threshold value of IL-18 for diagnosis of CTD-ILD was 294.07 pg/mL (sensitivity: 87.9%, specificity 83.6%, P < .05), and the optimal critical value of S100A8/A9 for diagnosing CTD-ILD was 303.77 ng/mL (sensitivity: 97%, specificity 70.5%, P < .05). The combined diagnosis of CTD-ILD by serum KL-6, IL-18, and S100A8/A9 had a sensitivity of 87.9% and a specificity of 95.1%, and the combined detection of the 3 markers could improve the diagnostic specificity. Serum KL-6, IL-18, and S100A8/A9 are valuable in the diagnosis of CTD-ILD. They can be used as serologic indicators for diagnosing ILD.