Incidence of venous thromboembolism in young Swedish women and possibly preventable cases among combined oral contraceptive users

Background. We wanted to study the incidence of venous thromboembolism (VTE), acquired risk factors of VTE and preventable cases among users of combined oral contraceptives (COCs). Methods. All women aged 15-44 years, (n=24373) living in the county of Jamtland, Sweden, between 1991 and 2000, constituted the study base in a retrospective case-reference study. Women with VTE were identified through hospital registers and interviewed by telephone. The utilization of COCs according to age was obtained from a prospective prescription database, and data from national health databases were used. Results. Of 88 women with first-time VTE, 43 (49%) were COC users and 13 (15%) were pregnant. All women had at least one known risk factor, and 51 (58%) women had combinations of risk factors. The total incidence rate of VTE per 100 000 women-years for all women were 36 (29-44), for nonusers 19 (12-25) for women using third generation COCs 115 (67-184), for women using other COCs 60 (37-83), and for women during pregnancy and postpartum 103 (55-177). Of the total 244 000 women-years represented, COC users constituted 24%, pregnant women 5%, and women with other acquired risk factors 5%. The corresponding incidence rates after excluding VTE cases with other acquired risk factors were 10 (6-14), 1.2 (0.14-4.4), 64 (29-121), 27 (13-48), and 59 (24-121), per 100 000 women-years. In 11 (26%) of the COC-related VTE cases, there were relative contraindications for use of COCs or lack of thromboprophylaxis in relation to surgery. Conclusion. We found a very low incidence of idiopathic VTE among young non-OC users. The incidence of VTE during pregnancy was only slightly higher than during COC use. It was considered that a significant part of COC-related VTE might have been avoided.