GDF15 and chronic obstructive pulmonary disease: Univariate Cox proportional risk analysis indicated that high HN (p = 0.001), the mMRC dyspnea score (p < 0.001), the Charlson index (p < 0.001), FEV1 (p = 0.001) and previous admission for COPD exacerbation (p < 0.001) were risk factors for hospitalization, whereas high GDF15 levels were not.