GSTM1 and anemia (phenotype): The PCr/ATP ratio remained significantly different between patients with and without ID after univariable adjustment for gender (adjusted post‐hoc p = 0.005), ethnicity (adjusted post‐hoc p = 0.005), Hb levels (adjusted post‐hoc p = 0.005), presence of anaemia (adjusted post‐hoc p = 0.002), presence of diabetes (adjusted post‐hoc p = 0.009), heart rhythm (adjusted post‐hoc p = 0.001), NYHA class (adjusted post‐hoc p = 0.02), and use of β‐blockers (adjusted post‐hoc p = 0.004).