Contact factor mediated fibrinolysis is increased by the combined oral contraceptive pill

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Abstract
Objective To study the fibrinolytic pathways and their relationship with the contact system in women using combined oral contraceptives (COCs).Design Serial plasma samples were collected from 18 women before treatment with COCs containing 30 μg oestrogen during treatment cycles 3 and 6, and 2 weeks after stopping treatment. Fibrinolysis was measured before and after dextran sulphate mediated contact activation using fibrin plates.Results Fibrinolysis increased significantly during cycles 3 and 6 (from 77% to 100% and 113%, respectively, PPPPPPConclusions The increase in fibrinolysis may be due to increased levels of Factor XII and prekallikrein without a corresponding increase in their natural inhibitors (C1‐esterase inhibitor and alpha‐2‐macroglobulin). A parallel increase in the intrinsic pathway of coagulation may be limited by elevated alpha‐1‐antitrypsin at the level of activated Factor XI. The increase in fibrinolysis caused by oral contraceptives may balance any potential thrombotic risk due to increased fibrinogen or vitamin K dependent coagulation factors.