We thank Mou and Pan for their interesting and constructive correspondence [1] regarding our meta-analysis on fractional exhaled nitric oxide (FeNO) in acute exacerbations of chronic obstructive pulmonary disease (AECOPD) [2]. Their letter addresses an essential point, namely that biomarkers in AECOPD should not be interpreted through a simplistic binary lens, because exacerbations represent a biologically heterogeneous clinical syndrome rather than a single inflammatory entity.
