IntroductionAspirin resistance (AR) has been reported to present after coronary artery bypass graft causing saphenous vein graft failure. We aimed to investigate the factors that affect the anti-platelet effect of aspirin after off-pump coronary artery bypass surgery (OPCAB). Materials and methodsThirty OPCAB candidates were successively recruited. Platelet count, platelet aggregation, reticulated platelet (RP), platelets' cyclooxygenase (COX)-1 and COX-2 expressions were determined during the peri-operative period. Besides, 10 healthy volunteers were enrolled to determine the onset of the anti-platelet effect of aspirin as comparison. Results and conclusionsThe arachidonic acid-induced platelet aggregation (PLAA) decreased to <20% within 8h after taking 100mg aspirin in healthy volunteers. However, in the OPCAB patients, PLAA levels remained over 20% in 16 (53.3%) patients after resuming aspirin for 24h. The surgical bleeding volumes were higher in the AR patients compared to the normal responders (512.5±192.8 vs. 314.3±94.9, p=0.002). The platelet count on Day 8, RP proportions on Days 1, 4, 8, and COX-2 level on Day 4 were significantly increased compared to their baseline levels in AR group but not in AS group. Platelet count on Day 8, RP proportion and COX-2 on Day 4 were all significantly higher in AR group than those in AS group. The surgical bleeding volume and COX-2 over-expression were predictors of post-OPCAB AR. As a conclusion, the inability of aspirin to inhibit the COX-2 created by increased RP would account for the post-OPCAB AR.