Serum KL-6 level is a useful biomarker for evaluating the severity of coronavirus disease 2019

Respir Investig. 2020 Nov;58(6):440-447. doi: 10.1016/j.resinv.2020.07.004. Epub 2020 Aug 21.


Background: Coronavirus disease 2019 (COVID-19) is currently spreading worldwide. This study examined whether serum Krebs von den Lungen-6 (KL-6) level is a useful biomarker for evaluating the severity of COVID-19.

Methods: We retrospectively examined patients diagnosed with COVID-19 at the Japanese Red Cross Medical Center between February 1, 2020, and May 15, 2020. Patients were divided into four categories based on clinical and radiological findings: mild, moderate, severe, and critical. Patients who presented with a mild or moderate illness and patients who started with or worsened to a severe or critical illness were classified as the non-severe and severe groups, respectively. The two groups were compared for patient characteristics, including serum KL-6 levels. Receiver operating characteristic curves were used to define the optimum cut-off value of serum KL-6 level to evaluate COVID-19 severity.

Results: A total of 54 patients were enrolled, including 33 in the non-severe group and 21 in the severe group, of which four died. Compared with those in the non-severe group, more patients in the severe group were significantly older and had comorbidities. Serum KL-6 levels were significantly higher in the severe group than in the non-severe group both at diagnosis (median, 338 U/mL) and at peak levels within one week after diagnosis (median, 781 U/mL) (both p < 0.001). Serum KL-6 value at peak level (371 U/mL) was used as the optimal cut-off to evaluate disease severity (sensitivity, 85.7%; specificity, 96.6%).

Conclusions: Serum KL-6 levels were significantly elevated in severe COVID-19 and is useful for evaluating its severity.

Keywords: Biomarker; Coronavirus disease 2019; Krebs von den Lungen-6; Pneumonia; Viral infection.

MeSH terms

  • Biomarkers / blood
  • COVID-19* / diagnosis
  • Humans
  • Mucin-1 / blood*
  • ROC Curve
  • Retrospective Studies


  • Biomarkers
  • MUC1 protein, human
  • Mucin-1