The activated factor X-antithrombin III reaction rate: a measure of the increased thrombotic tendency induced by estrogen-containing oral contraceptives in rabbits.

Estrogen-containing compounds are thrombogenic in man. The extent of thrombosis is dose-related, but a measure of this thrombogenicity is not currently available. Evidence is presented, using an animal model of venous thrombosis, that impaired Xa inhibitory activity (the rate of reaction between Xa and antithrombin III) correlates with the development of thrombosis initiated by thrombin in rabbits receiving an oral contraceptive compound. The responses in the clotting assay and in the extent of thrombosis are dose-related. The hypercoagulable state induced by oral contraceptives can be completely reversed by trace amounts of heparin.