In this single-center, retrospective, cohort study involving 271 patients who underwent elective total knee arthroplasty, we observed that the only baseline variable that had a significant association with postoperative anemia (defined as postoperative Hb < 9 g/dL) was a lower preoperative hemoglobin level (10.3 ± 0.6 vs. 13.1 ± 1.3). Here, GSTM1 is linked to anemia (phenotype).