MPO and asthma: Although the relations between serum YKL-40 levels and AAV variants, ANCA types, and involved organs are unclear, the presence of MPO-ANCA (or P-ANCA) could be also relevant to this observation, as a previous study discovered that sputum YKL-40 is correlated with serum YKL-40, as well as sputum MPO, vascular endothelial growth factor, IL-6, and neutrophil count in patients with asthma [45].