Association of Serum KL-6 with Functional Impairment in Patients with Immune-Mediated Interstitial Lung Disease

Background and Objectives: Krebs von den Lungen-6 (KL-6) is a promising biomarker of interstitial lung disease (ILD), but data in immune-mediated ILD remain limited. We aimed to evaluate the clinical, functional, and radiological correlates of serum KL-6 levels in a real-world cohort of patients with immune-mediated ILD. Materials and Methods: We conducted a retrospective, cross-sectional, single-center study including adults with multidisciplinary follow-up for immune-mediated ILD and at least one serum KL-6 determination between January 2023 and September 2025. Clinical, laboratory, radiological, and pulmonary function data closest to the index KL-6 measurement were collected. KL-6 was analyzed as both a continuous variable and a categorical variable using a predefined cutoff of 500 U/mL. Correlation analyses and multivariable logistic regression were performed to identify factors independently associated with elevated KL-6 levels. Results: A total of 112 patients were included; 72.3% were women, with a median age of 70 years (IQR 63–77). The most frequent underlying diseases were systemic sclerosis (30.4%), rheumatoid arthritis (25.9%), and inflammatory myopathy (14.3%). Median KL-6 concentration was 770.5 U/mL (IQR 437.3–1148.5). Patients with FVC < 80% predicted and DLCO < 60% predicted had significantly higher KL-6 levels than those with better lung function (p = 0.001 and p = 0.004, respectively). KL-6 levels correlated inversely with DLCO (% predicted) (ρ = −0.388, p < 0.001) and FVC (% predicted) (ρ = −0.328, p < 0.001) but not with the presence of radiological fibrosis. In the multivariable analysis, lower DLCO (% predicted) (OR 0.919, 95% CI 0.884–0.955; p < 0.001) and older age (OR 1.063, 95% CI 1.014–1.115; p = 0.012) were independently associated with elevated KL-6 levels. Conclusions: Elevated serum KL-6 concentrations were associated with impaired gas transfer as measured by DLCO % predicted in this cross-sectional cohort. Longitudinal studies are required to determine whether KL-6 can predict functional decline, disease progression, or clinical outcomes.