Oral contraceptives and surgery: Reduced antithrombin and antifactor Xa levels without postoperative venous thrombosis in low-risk patients

The effect of preoperative treatment with an estrogen containing oral contraceptive on the incidence of postoperative thrombosis (detected with ¹²⁵I-fibrinogen leg scanning) was examined in 221 patients aged 18-49 years who had elective or emergency general intra-abdominal or gynecologic surgery, 99 of whom were taking an oral contraceptive before their operation. Eighty-five percent of the contraceptive treated patients had taken their preparation until 1-7 days before surgery and the rest until within a month of surgery; 81% were taking preparations containing 30 or 50 μg estrogen. Postoperative thrombosis developed in 1 of the 122 patients who were not taking a contraceptive and in none of the 99 contraceptive treated patients. As the 95% confidence limits for the 0% thrombosis rate observed in the contraceptive treated patients are 0-3.7%, the added risk of postoperative thrombosis attributable to oral contraceptive treatment before surgery is very small in young women who are otherwise at low risk of thrombosis. Plasma antithrombin level (by immunoassay), antithrombin activity (by chromogenic substrate assay), and antifactor Xa activity (by clotting assay), were measured before surgery in 81 patients, 42 of whom had been taking an oral contraceptive. Contraceptive treatment significantly reduced all 3 assay results. Its most marked effect was on antifactor Xa activity, which was reduced by contraceptive treatment from 103 +/- 24% (range 68-172%) to 81 +/- 27% (range 24-155%; with <80% activity in 54%, and <50% activity in 15% of patients). These results suggest that reduced preoperative antifactor Xa activity has a low specificity for thrombosis after moderately extensive surgery in young, otherwise fit, oral contraceptive treated women. This test is therefore unlikely to be a clinically useful predictor for postoperative thrombosis in this group of patients.