Platelets obtained from women using estrogen-progesterone oral contraceptives are evaluated for activity as cofactor for Factor Xa-catalyzed prothrombin activation. Donor platelet-rich plasma is diluted into a standard platelet-poor plasma to minimize variations in plasma Factor X and prothrombin concentrations. Activation is initiated with Russell's viper venom, arrested at intervals with EDTA, and residual prothrombin is measured. Compared with controls, platelets from women using oral contraceptives promote an increase in both rate and extent of prothrombin activation, with control and test groups falling into non-overlapping populations. The increased prothrombin activation correlates with an increase in antithrombin III consumption. Enhanced antithrombin III consumption explains the decreased serum antithrombin III levels among women using oral contraceptives.
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