The influence of oral contraceptives on activated factor X inhibitor (XaI)-activity: a prospective study

A prospective study of activated factor X inhibitor (XaI)-activity has been performed in women randomly allocated to contraceptive pills containing 30 micrograms + 150 micrograms (63 women) or 50 micrograms + 250 micrograms (44 women) ethinyl oestradiol and levonorgestrel, and a reference group of 35 women using other contraceptive methods. The XaI-activity was measured six times during 1 year of observation. There was a significant reduction of XaI-activity after 1 month of oestrogen consumption but there was no significant difference between 30 and 50 micrograms users. The maximal reduction during the observation period was 12% in both groups. There was positive correlation between the pretreatment value and the maximal reduction in the same patient, but the activity did not fall below the normal range (75-125% of normal human plasma).