A multicentre study of coagulation and haemostatic variables during oral contraception: variations with geographical location and ethnicity. Task Force on Oral Contraceptives--WHO Special Programme of Research, Development and Research Training in Human Reproduction.

A comparative study of the effects of combined oral contraceptives (OC) on coagulation and fibrinolytic variables using standardized laboratory technique and methodology has been performed in Dublin (Ireland), Salvador (Brazil), Santiago (Chile) and Singapore. Of 777 entrants to the study, 622 were randomly allocated to receive one of four different OC formulations. The remainder did not opt for OC. The progestogenic component was levonorgestrel (LNG) in three of the OC formulations and norethisterone acetate (NEA) in the fourth. Results for the three LNG user groups were pooled. The changes in haematological variables observed over 12 months in the LNG and NEA users were examined in relation to the changes seen in the women not on OC. Women in Salvador differed markedly from those in the other three centres, in showing no acceleration of the prothrombin time and no increase in either fibrin plate lysis or plasminogen following the use of OC. After adjusting the findings in OC users for those in non-users, significant differences in response between centres were also detected for activated partial thromboplastin time (accelerated only in Dublin and Santiago), factor VII activity (increased mainly in Salvador and Santiago) and fibrinogen (for which the most marked changes were an increase in Dublin and a decrease in Salvador). This variability between centres in the effects of OC on coagulation and fibrinolysis suggests that OC administration in different populations may not carry equal thrombotic risks.