GDF15 and chronic obstructive pulmonary disease: Comparing subjects by the GDF-15 levels, those in COPD (1285 pg/mL) showed trend of the highest values among the subjects (p < 0.05) and GDF-15 levels in COPD were higher than those in healthy subjects and asthmatics by post-hoc analysis of differences between the specific groups (830 pg/mL; p < 0.005, 793 pg/mL; p = 0.08, respectively).