GDF15 and chronic obstructive pulmonary disease: Moreover, serum GDF15 levels were negatively correlated with BMI (r = −0.562, p < 0.001), mid-arm circumference (r = −0.505, p < 0.001), calf circumference (r = −0.490, p < 0.001), total protein (r = −0.486, p < 0.001), ALB (r = −0.445, p < 0.001), and prognostic nutritional index (r = −0.276, p = 0.001), and positively correlated with C-reactive protein (r = 0.318, p < 0.001), COPD assessment test score (r = 0.286, p = 0.001), modified medical research council classification (r = 0.310, p < 0.001), and global initiative for chronic obstructive pulmonary disease grade (r = 0.177, p = 0.035).