Thrombin-antithrombin III complex (TAT) is a good marker of intravascular thrombin generation. Oral anticoagulant therapy can be expected to be effective when intravascular thrombin generation is suppressed to an adequate range. TAT levels in the blood plasma from 61 patients receiving warfarin therapy and 12 patients prior to the initiation of warfarin therapy were determined. The degree of anticoagulation was monitored by the one-stage prothrombin time method (PT). Although PT levels were higher than the control range (more than 35%) in 14 of the patients receiving warfarin therapy, there was no significant difference in mean plasma TAT levels between these 14 patients and the 46 patients whose PT levels were within the control range. The mean TAT level in all patients receiving warfarin therapy (1.76±0.46ng/ml) was significantly lower than that in the patients before anticoagulation (5.19±1.97ng/ml) and was nearly the same as the normal control (2.00±0.58ng/ml). No significant relationship was observed between the levels of TAT and PT (%, INR). In several patients receiving warfarin therapy, serial changes in plasma levels of TAT and PT were followed. It was noted that plasma TAT levels always remained within the normal range even if PT (%) deviated from control range, while both PT (%) and TAT levels shifted in a similar direction. These results suggest that conventional PT may not always be an appropriate index to monitor oral anticoagulant therapy, and that the optimal range of anticoagulation for hypercoagulable states should be set according to the plasma levels of TAT.
