MMP9 and chronic obstructive pulmonary disease: In COPD patients, this review article is based on a study by Brajer et al. 2008 and Erlewyn-La-jeunesse et al. 2008 also showed an increase in serum MMP-9 levels, which was more negatively correlated with the Tiffeneau-Pinelli index (FEV1/FVC ratio).