Background/Purpose: Interstitial lung disease (ILD) is one of serious organ involvements in idiopathic inflammatory myositis. Krebs von den Lungen-6 (KL-6) is an antigen produced primarily by regenerating type II pneumocytes and has been known as a biomarker in ILD, although whether it can be used to assess myopathy-associated ILD is unclear. The aim of this study is to elucidate the usefulness of KL-6 in myositis-associated ILD.

Methods: We reviewed consecutive patients with myositis-associated ILD in our institution between 2002 and 2017 and enrolled those with serum KL-6 at diagnosis available. We divided the patients into two groups who relapsed and did not (relapse group and non-relapse group), and compared serum KL-6 levels and its time-course changes during the observation period between 2 groups.

Results: Fifty-eight patients with myositis-associated ILD were included in the analysis. Median age at diagnosis was 51 years old, and female was 60%. Diagnosis consisted with 13 (22%) with polymyositis, 22 (38%) with dermatomyositis and 23 (40%) with clinically amyopathic dermatomyositis. Of those patients, 21 (36%) relapsed and 37 (64%) did not. Median period from diagnosis to the last visit in the non-relapse group was 27 months, and from diagnosis to the relapse in the relapse group was 26 months. Median KL-6 levels at diagnosis were significantly higher in the relapse group than the non-relapse group (1870 vs 935 U/mL, p = 0.003). A receiver operating characteristic curve identified baseline KL-6 of 1359 U/mL as a significant indicative level for futuristic relapse. When we followed the relapse group until the last visit (median period was 82 months), 6 patients experienced second or third relapse. The time-course change in serum KL-6 in the relapse group showed remarkable decrease at remission and increase at relapse in each time (Figure). The range of fluctuation of KL-6 levels was significantly wider in the relapse group than in the non-relapse group (866 vs 259 U/mL, p < 0.001), suggesting KL-6 levels in the non-relapse group was stable. The increase in KL-6 levels of 625 U/mL could be a cut-off level for relapse by receiver operating characteristic analysis.

Conclusion: Serum KL-6 is a useful biomarker for assessing activity of myositis-associated interstitial lung disease.