Sputum biomarkers relevant to COPD diagnosis can be divided into two main groups: those specific to neutrophilic COPD (the eosinophil-to-neutrophil ratio, elevated levels of IL-8, IL-1β, and TNFα) and those specific to its eosinophilic variant (eosinophil cationic protein (ECP) and periostin, an IL-13-induced protein) [12,13,14]. Here, POSTN is linked to chronic obstructive pulmonary disease.