MMP9 and chronic obstructive pulmonary disease: Serum MMP‐9 levels were significantly higher in the COPD‐S group (258.68 ng/ml) than in the healthy‐S group (156.95 ng/ml; P = 0.04 versus COPD‐S) and the COPD‐NS group (111.38 ng/ml; P = 0.05 versus COPD‐S; Fig. 1A).