Diagnostic utility of serum KL-6 levels in acute respiratory failure with bilateral pulmonary infiltrates

Background: Serum levels of KL-6 are useful biomarkers for diagnosis of interstitial lung diseases (ILDs). However, its diagnostic utility in the setting of acute respiratory failure (ARF) is unknown. Aims and Objectives: We investigated how high serum KL-6 levels contributed to the differential diagnosis of ARF. Methods: We retrospectively examined consecutive 138 patients who were admitted to our department for the treatment of ARF with bilateral pulmonary infiltrates and were evaluated for serum KL-6 levels, from April 2015 to July 2018. Patients with apparent preexisting pulmonary fibrosis were excluded. Results: KL-6 values were high (≥ 500 U/mL) in 52 patients. Pulmonary diseases responsible for high KL-6 levels were detected in all patients of the high KL-6 group; ILDs associated with connective tissue diseases in 11 patients, latent chronic ILDs in 10 patients, drug-induced ILDs in 9 patients, pneumocystis pneumonia in 5 patients, radiation pneumonitis in 4 patients, and other etiologies in 13 patients. These KL-6-elevating diseases were the direct causes of ARF in 40 (77%) patients in the high KL-6 group compared to 22 (26%) patients in the normal KL-6 group (p Conclusions: In ARF with bilateral pulmonary shadows and no apparent chronic fibrotic lesions, elevated serum KL-6 levels could be a marker of causative diseases other than bacterial pneumonia that often require steroid therapy.