Since this study is not randomized, the patients receiving clopidogrel preoperatively may not have the same peri-operative risk of excessive bleeding than those who do not. Therefore, we will perform separate multivariable risk adjustment to analyze the perioperative risk associated with clopidogrel treatment. For each patient, a propensity score, indicating the likelihood of having clopidogrel, will be calculated. The propensity score-matched analysis will allows us to determine whether preoperative clopidogrel therapy is independently associated with an increased risk of excessive postoperative bleeding. All preoperative variables as well as second-order interaction terms will be entered into the model. Calibration and discrimination of the final logistic model will be assessed using the Hosmer-Lemeshow statistics and the receiver operating characteristic curve, respectively. Comparison of mean bleeding between several groups (according to propensity score quintiles) will be performed using analysis of variance.