MMP9 and chronic obstructive pulmonary disease: When adjusted for age and pack‐years, both MMP‐9 and proMMP‐9/NGAL levels remained significantly associated with smoking status in COPD when compared with the healthy‐S group (MMP‐9: P adjusted for age = 0.001, and P adjusted for age and pack‐years = 0.003; proMMP‐9/NGAL: P adjusted for age = 0.014, and P adjusted for age and pack‐years = 0.017; Table S1).