In contrast, all the studied parameters were significantly correlated in the COPD‐S group (Fig. 2D–F): there were positive correlations between MMP‐9 and the proMMP‐9/NGAL complex (r = 0.84, P < 0.0001; Fig. 2D), between MMP‐9 and NGAL (r = 0.56, P < 0.0001; Fig. 2E) and between NGAL and the proMMP‐9/NGAL complex (r = 0.53, P < 0.0001; Fig. 2F). This evidence concerns the gene MMP9 and chronic obstructive pulmonary disease.