The prevalence of anemia in patients undergoing TKA and THA is estimated at 25% [25] There is ample evidence that a low pre-operative hemoglobin (Hb) level increases transfusion requirement, infection risk, and LOS after joint arthroplasty [25-27] The absolute blood loss itself during surgery does not seem to delay discharge [21], reinforcing the notion that it may well be the patients starting point (the pre-operative Hb level) which is a more important determinant of LOS. Here, GSTM1 is linked to anemia (phenotype).