Background: Many observational studies have described an association between perioperative transfusion and postoperative infection. Detection of such a relationship may depend on which variables are considered as potential confounders of the association under study. However, most reports have not considered risk factors for postoperative infection at specific sites as possible explanations for the observed relationship.
Study design and methods: The records of 492 patients undergoing elective colorectal cancer resection at the Massachusetts General Hospital between January 1992 and December 1994 were reviewed. The probability of infection in association with transfusion was calculated with and without adjustment for the effects of chronic systemic illness, number of days with urinary catheter, endotracheal intubation, impaired consciousness, and specific risk factors for wound infection. Postoperative infection at any site and infections at specific sites were analyzed as separate outcomes.
Results: After adjustment for the effects of the variables listed above, allogeneic transfusion was not associated with postoperative infection at any site (p = 0.407). Only a specific association of transfusion with wound infection could be detected. However, in an analysis that adjusted for the effects of only the 18 confounders considered by previous authors, transfusion was the most significant predictor of infection. In that analysis, the risk of postoperative infection increased by 14 percent per unit of red cells transfused (p < 0.001).
Conclusion: The overall adverse relationship between transfusion and infection reported by previous observational studies may have been due to an incomplete consideration of the variables that confound that association. This finding may help explain the disagreement between the conclusions of recent large, randomized, controlled trials (which failed to detect a deleterious transfusion effect) and the earlier observational studies.