BACKGROUND: Thromboelastogram (TEG) is regarded as a method by which coagulation can be measured during surgery. Off pump coronary artery bypass (OPCAB) surgical patients are routinely placed on aspirin. But conventional TEG could not detect platelet dysfunction by antiplatelet agent. We used modified TEG (addition of heparin and platelet agonists) to determine whether this modified TEG could assess the platelet dysfunction by aspirin in OPCAB surgical patients and healthy volunteers. METHODS: After institutional review board approval and consent, platelet function of 10 OPCAB patients (group 1) and 10 healthy subjects (group 2: before aspirin, group 3: after aspirin) were measured using modified TEG. In each group, TEG parameters (R, K time and MA) were analyzed using paired t-test and one way ANOVA was used to determine the difference between groups. RESULTS: In group 1 (OPCAB patients) and group 2 (healthy subjects, before aspirin), the R and K time were increased significantly with the addition of heparin and then decreased subsequently with the platelet agonists (ADP or collagen) in the presence of anticoagulation (heparin). MA showed a decrease in both groups. This compares with no significant difference in all parameters in group 3 (healthy subjects, after aspirin) with the addition of heparin, ADP and collagen. There were no significant differences in each TEG parameter between groups. CONCLUSIONS: This study suggested that aspirin medication obliterated the effect of anticoagulation and platelet agonists in modified TEG. However modified TEG does not provide a comprehensive and sensitive reflection of platelet inhibition by aspirin. TEG should be supplemented by other methods of platelet function assessment.