Which anticoagulant does not cause abnormal thrombin time (TT)?

Aspirin does not cause abnormal thrombin time (TT) because its primary mechanism of action is the irreversible inhibition of cyclooxygenase (COX-1 and COX-2), which affects platelet function by preventing the formation of thromboxane A2, a promoter of platelet aggregation. As a result, while aspirin influences the clotting process, its effect is mainly on platelet activity rather than directly on the coagulation cascade.

In contrast, heparin, warfarin, and direct thrombin inhibitors directly impact the coagulation pathway, which is assessed by thrombin time. Heparin enhances the activity of antithrombin III, which inhibits thrombin and factor Xa, resulting in prolonged TT. Warfarin, by inhibiting vitamin K-dependent clotting factors (such as factors II, VII, IX, and X), also affects the normal functioning of coagulation, leading to an increased TT. Direct thrombin inhibitors, as their name suggests, work by inhibiting the activity of thrombin itself, directly prolonging the TT.

Thus, aspirin stands out among these options for its lack of an effect on thrombin time, making it the correct answer to the question.