Travel-Related Venous Thrombosis: Results

A B S T R A C T
Background
Recent studies have indicated an increased risk of venous thrombosis after air travel.
Nevertheless, questions on the magnitude of risk, the underlying mechanism,and modifying
factors remain unanswered.
Methods and Findings
We studied the effect of various modes and duration of travel on the risk of venous
thrombosis in a large ongoing case-control studyon risk factors for venous thrombosis in an
unselected population (MEGA study). We also assessed the combined effect of travel and
prothrombotic mutations, body mass index, height, and oralcontraceptive use.
Since March 1999, consecutive patients younger than 70 y with a first venous thrombosis
have been invited to participate in the study, with their partners serving as matchedcontrol
individuals. Information has been collected on acquired and genetic risk factors for venous
thrombosis. Of 1,906 patients, 233 had traveled for more than 4 h in the 8 wk preceding theevent. Traveling in general was found to increase the risk of venous thrombosis 2-fold (odds
ratio [OR] 2.1; 95% confidence interval [CI] 1.5–3.0). The risk of flying was similar to the risks oftraveling by car, bus, or train. The risk was highest in the first week after traveling. Travel by car,
bus, or train led to a high relative risk of thrombosis in individuals with factor V Leiden (OR8.1;
95% CI 2.7–24.7), in those who had a body mass index of more than 30 kg/m2 (OR 9.9; 95% CI
3.6–27.6), in those who were more than 1.90 m tall (OR 4.7; 95% CI 1.4–15.4), and in those whoused oral contraceptives (estimated OR . 20). For air travel these synergistic findings were
more apparent, while people shorter than 1.60 m had an increased risk of thrombosis after air
travel(OR 4.9; 95% CI 0.9–25.6) as well.
Conclusions
The risk of venous thrombosis after travel is moderately increased for all modes of travel.
Subgroups exist in which the risk is highly increased.