GSTM1 and anemia: However, since DLCO is markedly affected by blood haemoglobin (MacIntyre et al., 2005; Tsai et al., 2013) and CO–Hb binding is pivotal in CO transfer, these findings may be confounded by the presence of anaemia, polycythaemia and erythrocytosis (Dumitrescu et al., 2017), of which anaemia is prevalent among most PH patients and is associated with poor clinical outcomes, increased disease severity, morbidity and mortality (Krasuski et al., 2011; Sonnweber et al., 2020).