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Thirteen asymptomatic women taking oral contraceptives (OCs) were compared with normal subjects. Platelet aggregation and serotonin (tagged with carbon 14) release with adenosine diphosphate, epinephrine, and collagen did not differ from controls. Activation of the intrinsic coagulation pathway was evaluated by measurements of factor XII, prekallikrein, and kallikrein inhibitors. Women taking OCs had normal factor XII levels, moderately elevated prekallikrein levels, and decreased kallikrein inhibitor levels, a pattern not consistent with activation of the intrinsic pathway. The concentration of heavy molecular weight fibrinogen derivatives (HMWFD) was significantly elevated (P less than .001). In contrast, fibrin and fibrinogen degradation products in serum were lower than normal (P less than .05). The increased HMWFD concentration may reflect activation of the coagulation system not mediated by factor XII activation nor potentiated by decreased antithrombin III. Thus, plasma coagulation abnormalities rather than platelet changes may be the factor that predisposes women taking OCs to thromboembolic disorders.
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