We recommend that all patients with a subnormal hemoglobin level should have a repeated simple biochemical standard blood test including the measurements of Hb, MCV, eGFR, Ferritin, s-TfR (or se-Fe and TIBC), CRP, B12 and folate, to indicate whether the patients have an explained or unexplained anemia. This evidence concerns the gene GSTM1 and anemia (phenotype).