GSTM1 and stage 5 chronic kidney disease: HD: Haemodialysis, SC: Subcutaneous, IV: Intravenous, Hb: Haemoglobin, rHuEPO: Recombinant Human Erythropoietin, ESRD: End-stage Renal Disease, AE: Adverse events, EPO: Erythropoietin, PRCA: Pure Red Cell Aplasia