Recent advances in understanding clotting and evaluating patients with recurrent thrombosis

Significant advances have been made in defining the regulatory mechanisms that control blood clotting. These are reviewed, with special attention to the functions of the natural inhibitors antithrombin III, protein C, and protein S. Congenital deficiencies of these inhibitors as well as acquired abnormalities, such as defective fibrinolysis, and their role in promoting thrombosis are also discussed, as are thrombotic complications of pregnancy. Pregnancy decreases levels of protein S to 40% to 50% of normal levels. The decrease occurs early in pregnancy and persists into the postpartum period; it appears to be a hormonal rather than a dilutional effect. It is not known whether the thrombotic risk associated with pregnancy is increased in women who are congenitally deficient in protein S. Oral contraceptives decrease levels of protein S by about 20%. Women with a personal or family history of thrombosis should be evaluated for predisposing conditions before they start an oral contraceptive, as should women taking oral contraceptives who develop deep venous thrombosis.