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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.

A comparative study of coagulation and fibrinolytic laboratory parameters was undertaken in four countries (Salvador, Brazil; Singapore; Santiago, Chile and Dublin, Ireland) among apparently healthy women of reproductive age. A continuous external quality control scheme of the laboratory measurements was employed to permit comparison among centres. Significant and consistent differences were found between the four centres. In Dublin, the prothrombin time and activated partial thromboplastin time (APTT) were accelerated, and the specific factor assays showed more activity, whereas the antiprotease levels were higher than in the other centres. In Salvador, a contrasting tendency was found with longer prothrombin times and APTT and lower Factor VII and antiprotease levels. The results from the other two centres were approximately midway between these two extremes. The study has revealed important differences in the coagulation and haemostatic tests between women from widely diverse geographical areas. It is not certain whether these are due to ethnic, nutritional or economic factors but they may be related to the apparent varying incidence of thrombosis in these ethnic groups.