P OSTOPERATIVE INFECTIONS result from a com- plex interaction of three main groups of fac- tors: Bacteria (exogenous or endogenous), the surgical technique, and host defense mecha- nisms. Thus, identifying single determinant factors has proved difficult, and this difficulty applies specifically to the role of host defense mechanisms, the major impaired physiological response in surgical patients. MAGNITUDE OF TRANSFUSION- ASSOCIATED INFECTION RISK Research carried out during the mid-1980s and early 1990s demonstrated that periopera- tive blood transfusion is a risk factor for the de- velopment of postoperative infections, both in trauma patients and in patients undergoing surgery, mostly for gastrointestinal cancer. Since then, studies in patients with colorectal cancer have shown repeatedly that blood trans- fusion — mostly given intraoperatively—is a significant risk factor for postoperative infec- tion. In a recent survey of 2,809 colorectal re- sections [1], the odds ratio (OR) for postopera- tive infection was 5.3 to 6.2 if blood transfusion was given. Moreover, transfusion was the sin- gle most powerful risk factor, as the remaining ones identified by multivariate analysis (i.e., American Society of Anesthesiologists score, incision contamination, ostomy, male sex, SURGICAL INFECTIONS Volume 7, Supplement 2, 2006 © Mary Ann Liebert, Inc. Blood Transfusions and Postoperative Infections in Patients Undergoing Elective Surgery ANTONIO SITGES-SERRA, JOAN J. SANCHO INSENSER, and ESTELA MEMBRILLA ABSTRACT Background: Postoperative infections result from the interactions of bacteria, the surgical tech- nique, and host defense mechanisms. Thus, identifying single determinant factors has proved difficult. Magnitude of the risk: In a recent survey of 2,809 colorectal resections, transfusion was the single most powerful risk factor for postoperative infection. In patients undergoing primary hip or knee prosthesis insertion, the transfusion of allogeneic blood increased the risk of a deep-seated infection by a factor of 12. Mechanisms: Several host defense mechanisms are impaired by blood products. The initial hypothesis incriminated the transfused white blood cells, but this paradigm has since been challenged. The effects of free serum iron, the blood storage time, and the presence in stored blood of bioactive substances such as inhibitors of metalloproteinase-1 may also be impor- tant. Conclusion: It is worth pursuing efforts to emphasize autologous blood transfusion and the reinfusion of shed blood as blood conservation strategies, as these practices reduce the risk of infectious complications Department of Surgery, Hospital del Mar, Barcelona, Spain. S-33