Absolute Risk of Clots Higher for Frequent Flyers Who Take Long Trips

LEIDEN, The Netherlands, Sept. 25 -- The risk of symptomatic venous thrombosis for healthy individuals is low after an airflight of four or more hours, but increases with more and longer flights, researchers reported.

In a survey of almost 9,000 business travelers, the researchers found that long flights tripled the risk of developing venous thrombosis, Suzanne C. Cannegieter, M.D., Ph.D., of Leiden University here, and colleagues, reported online in PLoS Medicine.

That rate amounted to a risk for one person out of more than 4,600 flights, they said.

However, the study included only healthy individuals with an average age of 40 and without previous deep-vein thromboses or pulmonary emboli, so the absolute risk might be higher in the general traveling population, the researchers cautioned.

Although the link between air travel and venous thrombosis was first noticed in the 1950s, the absolute risk of developing deep vein thromboses after a long flight has not been known, the researchers wrote.

Their findings emerged from a cohort study of 8,755 travelers who were followed from January 2000 through December 2005 for a total of 38,910 person-years, during 6,872 of which they were exposed to a long flight, defined as more than four hours in duration.

In the follow-up period, 53 thromboses occurred, of which 22 occurred within eight weeks of a long flight.

This led to an incidence rate of 3.2 per 1,000 person-years, as compared with 1.0 per 1,000 person-years for 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 researcher reported, and was similar to the odds ratios found in most case-control studies.

The risk increased with exposure to more flights within a short time frame and with increasing duration of flights, the researchers said.

When someone was exposed to only one or two long flights, the rate per 1,000 person-years was 2.6, but it tripled after exposure to five or more long flights. With each extra flight, the risk increased 1.4-fold.

Expressed as risk per number of flights, the risk increased from 0.5 per 1,000 person years for flights under four hours to 5.91 per 1,000 person-years for flights lasting more than 16 hours.

The incidence was highest in the first two weeks after travel and gradually decreased to baseline after eight weeks, the researchers said.

The risk was particularly high for travelers younger than 30, women (especially those taking oral contraceptives), and individuals who were particularly short (pressure on the popliteal vein), tall (cramped position), or with a BMI over 25 kg/m².

However, the researchers said, because of the small number of cases, some confidence intervals were wide, indicating considerable uncertainty for these estimates. Nevertheless, they said, the findings indicate a need for adjustable seating.

The findings for younger travelers, they said, might be explained by the fact that younger travelers were most likely to be frequent travelers, thus explaining their absolute higher risk.

Discussing the study's limitations, the researcher noted that results were based on responses to a web-based questionnaire and the response rate of only 32% suggests that those who'd had a venous thrombosis may have been more likely to complete the questionnaire than those who did not.

Also, they noted, these results cannot be generalized to an older, less-healthy population or to individuals with a history of thrombosis.

However, they cautioned, their findings do not justify the use of potentially dangerous prophylaxis, such as anticoagulant therapy, for all long-haul air travelers, since this may do more harm than good.

Nevertheless, for some subgroups of people with a highly increased risk, the risk-benefit ratio may favor the use of prophylactic measures. Large randomized trials are required to assess who would benefit most and from which prophylactic measures, the researchers concluded.