Background/Purpose: To evaluate the association between serum levels of Krebs von den Lungen-6 (KL-6) and surfactant protein-D (SP-D) and the clinical manifestations and severity of interstitial lung disease (ILD) in patients with connective tissue disease (CTD).

Methods: Eighty patients with various CTDs were included, as follows: 33 with rheumatoid arthritis (RA), 19 with systemic lupus erythematosus (SLE), 10 with systemic sclerosis (SSc), 9 with Sjogren’s syndrome (SS), and 9 with inflammatory myositis. KL-6 and SP-D levels were measured using an enzyme-linked immunosorbent assay and defined as abnormal if KL-6 ≥ 500 U/mL and SP-D ≥ 110 mg/mL. All patients were simultaneously evaluated for parameters related to disease activity using laboratory tests and a pulmonary function test, and interstitial lung abnormalities (ILA) using chest computed tomography (CT). Patients were subclassified according to ILA score: 0 for no ILD, 1 for indeterminate ILD, 2 for mild ILD, and 3 for advanced ILD based on chest CT scans.

Results: In all, 29 patients had radiologically advanced ILD, 18 had mild ILD, 18 had indeterminate ILD, and 15 had no ILD. A higher ILA score was associated with more severe dyspnea, and decreased volume and percent of functional vital capacity, forced expiratory volume in 1 s, and diffusion capacity of carbon monoxide. As clinical manifestations, a higher ILA score was associated with a higher GAP index but not with the parameters of disease activity. A higher ILA score was associated with higher levels of KL-6 and SP-D and a higher percentage of subjects with abnormal levels, and this was more pronounced in SLE, SSc, and SS than in RA.

Conclusion: Serum levels of KL-6 and SP-D are associated with the radiological severity of ILD. Hence, these can serve as markers for ILD severity, especially in SLE, SSc, and SS.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/clinical-significance-of-kl-6-and-sp-d-as-serum-markers-for-interstitial-lung-disease-in-patients-with-connective-tissue-disease/