KL-6 is a long-term disease-activity biomarker for interstitial lung disease associated with polymyositis/dermatomyositis, but is not a short-term disease-activity biomarker.

We aimed to evaluate the usefulness of serum KL-6 for interstitial lung disease (ILD) with polymyositis/dermatomyositis (PM/DM). All consecutive and previously untreated adult patients with PM/DM who were admitted to our hospital from 2010 to 2015 were included. The associations between serum KL-6 levels and clinical information were retrospectively analyzed. Baseline serum KL-6 levels were significantly higher in patients with ILD than in those without ( = 41 and 15, respectively;  < .001). In the 14 patients whose ILD improved within 4 weeks post-treatment, their serum KL-6 levels did not significantly decrease at 2 weeks, 4 weeks, or 3 months post-treatment ( = 1.00, 1.00, and .83, respectively). Conversely, their serum KL-6 levels significantly decreased at 6, 9, and 12 months post-treatment ( = .01 in all comparisons). In the 12 patients whose ILD remained unchanged or deteriorated in 4 weeks post-treatment, only the difference between their serum KL-6 levels at 3 and 12 months was significant ( = .003). The present study validated the serum KL-6 as a diagnostic marker for ILD in PM/DM. However, serum KL-6 is not a short-term disease-activity biomarker for ILD with PM/DM, but it is a long-term disease-activity biomarker.