Risk of venous thrombosis with use of current low-dose oral contraceptivesis not explained by diagnostic suspicion and referral bias

Background: The magnitude of the relative risk of venous thrombosis caused by low-dose oral contraceptive use is still debated because previous studie s might have been affected by diagnostic suspicion and referral bias. Methods: We conducted a case-control study in which the effect of diagnosti c suspicion and referral bias was excluded. The study was performed in 2 di agnostic centers to which patients with clinically suspected deep vein thro mbosis of the leg were referred. History of oral contraceptive use was obta ined before objective testing for thrombosis. Young females with an objecti ve diagnosis of deep vein thrombosis were considered case patients, and tho se who were referred with the same clinical suspicion but who had no thromb osis served as control subjects. Participants were seen between September 1 , 1982, and October 18, 1995: 185 consecutive patients and 591 controls age d 15 to 49 years with a first episode of venous thrombosis and without mali gnant neoplasms, pregnancy, or known inherited clotting defects. Results: The overall odds ratio for oral contraceptive use was 3.2 (95% con fidence interval [CI], 2.3-4.5); after adjustment for age, family history o f venous thrombosis, calendar time, and center, the odds ratio was 3.9 (95% CI, 2.6-5.7). In the idiopathic group (120 patients and 413 controls, excl uding recent surgery, trauma, or immobilization), the odds ratio for oral c ontraceptive use was 3.8 (95% CI, 2.5-5.9); after adjustment, the odds rati o was 5.0 (95% CI, 3.1-8.2). Conclusions: In this study, in which patients and controls were subject to the same referral and diagnostic procedures, we found similar relative risk estimates for oral contraceptive use as in previous studies. We conclude t hat diagnostic suspicion and referral bias did not play an important role i n previous studies and that the risk of venous thrombosis with use of curre nt brands of oral contraceptives still exists.