Dysregulated immune response and abnormal repairment could cause secondary pulmonary fibrosis of
varying severity in COVID-19. The Krebs Von den Lungen-6 (KL-6) as a sensitive
marker reflects the degree of fibrosis and this study will focus on analyzing the evaluative efficacy and
predictive role of KL-6 in COVID-19 secondary pulmonary fibrosis. The study included total 289 COVID-19 patients who were divided into moderate (n=226) and severe
groups (n=63) according to the severity of illness. Clinical information such as inflammation indicators,
radiological results and lung function tests were collected. The time points of nucleic acid test were also
recorded. Furthermore, based on Chest radiology detection, it was identified that 80 (27.7%) patients
developed reversible pulmonary fibrosis and 34 (11.8%) patients developed irreversible pulmonary
fibrosis. Receiver operating characteristic (ROC) curve analysis shows that KL-6 could diagnose the
severity of COVID-19 (AUC=0.862) and predict the occurrence of pulmonary fibrosis (AUC = 0.741)
and irreversible pulmonary fibrosis (AUC=0.872).Importantly, the cross-correlation analysis
demonstrates that KL-6 rises earlier than the development of lung radiology fibrosis, thus also
illuminating the predictive function of KL-6. We set specific values (505U/mL and 674U/mL) for KL-6 in
order to assess the risk of pulmonary fibrosis after SARS-CoV-2 infection.The survival curves for days in
hospital show that the higher the KL-6 levels, the longer the hospital stay (P<0.0001).In conclusion, KL-6
could be used as an important predictor to evaluate the secondary pulmonary fibrosis degree for
COVID-19.