Serum KL‐6 concentrations as a novel biomarker of severe COVID‐19

Severe acute respiratory syndrome coronavirus 2–induced direct cytopathic effects against type I and II pneumocytes mediate lung damage. Krebs von den Lungen‐6 (KL‐6) is mainly produced by damaged or regenerating alveolar type II pneumocytes. This preliminary study analyzed serum concentrations of KL‐6 in patients with coronavirus disease (COVID‐19) to verify its potential as a prognostic biomarker of severity. Twenty‐two patients (median age [interquartile range] 63 [59‐68] years, 16 males) with COVID‐19 were enrolled prospectively. Patients were divided into mild‐moderate and severe groups, according to respiratory impairment and clinical management. KL‐6 serum concentrations and lymphocyte subset were obtained. Peripheral natural killer (NK) cells/µL were significantly higher in nonsevere patients than in the severe group (P = .0449) and the best cut‐off value was 119 cells/µL. KL‐6 serum concentrations were significantly higher in severe patients than the nonsevere group (P = .0118). Receiver operating characteristic analysis distinguished severe and nonsevere patients according to KL‐6 serum levels and the best cut‐off value was 406.5 U/mL. NK cell analysis and assay of KL‐6 in serum can help identify severe COVID‐19 patients. Increased KL‐6 serum concentrations were observed in patients with severe pulmonary involvement, revealing a prognostic value and supporting the potential usefulness of KL‐6 measurement to evaluate COVID‐19 patients' prognosis.