A funnel plot was constructed and plotted as log sample size against the respective odds ratio. The resulting funnel plot demonstrated no asymmetry and therefore met the criteria for lack of bias in article selection as described by Egger and Smith 8 (Fig. 4).
All point estimates of the individual odds ratios were greater than unity (1.0, or the odds ratio at which there is no increased risk of an event occurring), providing overwhelming evidence that undergoing ABT is associated with a significantly increased risk of postoperative bacterial infection. The common odds ratio for all publications included in this meta-analysis supports the understanding that ABT significantly increases the risk of postoperative bacterial infection in the surgical patient. Of interest is the finding that this risk is higher in the subset of reports that evaluated only trauma patients, suggesting that ABT may be an additive and frequently overlooked factor in the immunosuppression commonly reported in the postoperative trauma victim. 2,9
Logistic regression analysis has been used to define the relationship between ABT and infection while taking into consideration other frequently confounding variables, including age, shock, and wound contamination. This approach has been used by Tartter 10 for patients undergoing colon cancer surgery and has demonstrated a consistent highly significant relationship between ABT and postoperative bacterial infection rates. Other studies using similar statistical approaches also report that when additional confounding variables, including gender, Injury Severity Scores, length of surgery, and admission hematocrit are considered, ABT continued to be a significant independent predictor of an increased risk of postoperative infection in a dose-dependent fashion. 19 Other publications 1,2 report this dose-dependent effect of ABT on infection rates in humans, but because these studies 1,2 did not include a well-defined untreated control (nontransfused) group, they were not included in this meta-analysis. Although logistic regression analysis is a well-recognized statistical technique, clinical circumstances dictate that it is impossible to state with certainty that patients receiving ABT are in all respects similar to a nontransfused control group. Thus, these statistical results allow the statement to be made that ABT is associated with an increased risk of postoperative bacterial infection and may or may not be a primary causative factor.
The immunosuppressive effects of ABT may be long-standing. Evidence of immune dysfunction (reduced lymphocyte function, 36 reduced natural killer cell cytotoxicity scores, and helper/suppressor cell ratios 37) after prior blood transfusions has been reported to be demonstrable for several years. Other investigators report reduced T-cell and total lymphocyte counts in a transfused group when compared with a nontransfused group 18 months after transfusion. 38
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