GSTM1 and anemia: 17 studies presented the preoperative Hb or anemia data, and we found patients with transfusion had a lower preoperative Hb level [OR: −2.19, 95%CI: (−3.02, −1.36)] or higher prevalence of preoperative anemia [OR: 10.83, 95%CI: (7.23, 16.21)].