Oral contraceptives and low antithrombin-III activity.

2 independent investigations have shown that hereditary deficiency of antithrombin-III activity predisposes to thromboembolic disease in both males and females. It is also known that patients with thrombosis and embolism often have low antithrombin-III activity and that in individuals with repeated episodes of this type of clotting the antithrombin-III values tend to stay low over long periods of time. A preliminary report is made of a study undertaken after noting that women using oral contraceptives have very low antithrombin-III values. Antithrombin-III activity was expressed as antithrombin-III time with a short time indicating low antithrombin-III activity. The average value of normal individuals is 38.5 seconds compared with average values in patients with thromboembolism of 18.5 seconds. The average value in 11 women on oral contraceptives was 17.7 seconds. 2 of the women found to have the extremely low values of 13.8 and 14.9 seconds (indicating the near absence of antithrombin-III activity) subsequently developed thrombosis and embolism. The role of antithrombin-III or serum-antithrombin has not been established beyond all doubt. It is concluded that any further investigation into the effect of oral contraceptives on human blood coagulation should include antithrombin-III activity.