CHI3L1 and asthma: Even the most severe cases of asthma in our study had lower levels of serum YKL-40 than did controls in the other studies investigating serum YKL-40 in asthmatic individuals (15, 16, 42) Those with severe asthma in two other studies had almost a two-fold increase in serum YKL-40 compared with their counterparts in our study.