Population
Patients undergoing surgery (8 studies, n= 3,504,778).
Intervention
Preoperative blood transfusion.
Comparison
Without preoperative blood transfusion.
Outcome
Pooled analysis showed an association between preoperative blood transfusion and postoperative venous thromboembolism (VTE). In subgroup analyses, the positive association between preoperative blood transfusion and postoperative VTE still existed in studies with confounders adjustment.