Median serum level of YKL-40 was 206 ng/ml (95–431) in the COVID-19 group, 46 ng/ml (34–67) in the HS subgroup, whereas they were of 60 ng/ml (41–73) in the COPD and 73 ng/ml (42–91) in the ILD groups, respectively. Here, CHI3L1 is linked to chronic obstructive pulmonary disease.