Research Article: The Absolute Risk of Venous Thrombosis after Air Travel: A Cohort Study of 8,755 Employees of International Organisations

Date Published: September 25, 2007

Publisher: Public Library of Science

Author(s): Saskia Kuipers, Suzanne C Cannegieter, Saskia Middeldorp, Luc Robyn, Harry R Büller, Frits R Rosendaal, Eduardo L Franco

Abstract: BackgroundThe risk of venous thrombosis is approximately 2- to 4-fold increased after air travel, but the absolute risk is unknown. The objective of this study was to assess the absolute risk of venous thrombosis after air travel.Methods and FindingsWe conducted a cohort study among employees of large international companies and organisations, who were followed between 1 January 2000 and 31 December 2005. The occurrence of symptomatic venous thrombosis was linked to exposure to air travel, as assessed by travel records provided by the companies and organisations. A long-haul flight was defined as a flight of at least 4 h and participants were considered exposed for a postflight period of 8 wk. A total of 8,755 employees were followed during a total follow-up time of 38,910 person-years (PY). The total time employees were exposed to a long-haul flight was 6,872 PY. In the follow-up period, 53 thromboses occurred, 22 of which within 8 wk of a long-haul flight, yielding an incidence rate of 3.2/1,000 PY, as compared to 1.0/1,000 PY in individuals not exposed to air travel (incidence rate ratio 3.2, 95% confidence interval 1.8–5.6). This rate was equivalent to a risk of one event per 4,656 long-haul flights. The risk increased with exposure to more flights within a short time frame and with increasing duration of flights. The incidence was highest in the first 2 wk after travel and gradually decreased to baseline after 8 wk. The risk was particularly high in employees under age 30 y, women who used oral contraceptives, and individuals who were particularly short, tall, or overweight.ConclusionsThe risk of symptomatic venous thrombosis after air travel is moderately increased on average, and rises with increasing exposure and in high-risk groups.

Partial Text: In 1951, Jacques Louvel reported four cases of venous thrombosis following air travel [1]. More recently, several investigators have shown an association between air travel and venous thrombosis, with a 2- to 4-fold increased risk in most studies [2–8]. Two follow-up studies demonstrated a dose–response relationship between the occurrence of pulmonary embolism shortly after arrival at the airport and the distance travelled [9,10]. Still, the most relevant element, i.e., the absolute risk of symptomatic venous thrombosis after long-distance air travel, remains unknown. One follow-up study demonstrated an absolute risk of severe pulmonary embolism occurring shortly after arrival of 1/200,000 passengers [9], whereas another study showed a risk of fatal pulmonary embolism of 1.3 per million passengers [11]. Asymptomatic clots have been found in 1% to 10% of air travellers [12–14]. Hence, the absolute risk of symptomatic venous thrombosis after long-haul travel must lie between these extremes.

A total of 27,496 employees were invited to participate and 8,755 questionnaires were completed, yielding an overall response of 32% (range per organisation 15%–80%). General characteristics of the study population are shown in Table 1. More than half of the responders (n = 4,915, 56%) were men and the mean age was 40 y. The total follow-up time of participating employees was 38,910 PY, with a mean follow-up per participant of 4.4 y.

In this follow-up study, we found an overall absolute risk of symptomatic venous thrombosis of 1/4,656 passengers within 8 wk after flights longer than 4 h. This risk is equivalent to an IR of 3.2/1,000 PY. The risk was 3.2-fold increased compared to those who did not travel by air. The risk of venous thrombosis increased with exposure to several flights and longer duration of travel, and it decreased with time after a flight. It was particularly high in younger travellers, women (especially those taking oral contraceptives), individuals who were particularly short (<165 cm) or tall (>185 cm), and those with a BMI over 25 kg/m2, although due to the small number of cases, some confidence intervals were wide, indicating considerable uncertainty for the effect estimates.



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