In contrast to the physiological adaptation to hypoxia that should increase iron absorption and utilization with consequent compensatory Hb elevation [44], this pattern is observed only in a limited fraction of COPD patients: 40–50% of COPD patients instead develop iron deficiency, with anemia representing a predictive risk factor of worse outcome in 5–30% of cases [56,57]. Here, GSTM1 is linked to anemia (phenotype).