MedWire News: The risk for symptomatic venous thrombosis is generally increased after air travel, although only modestly, with further rises in risk seen with increasing exposure to air travel and in high-risk groups, study findings suggest.

Previous studies have indicated that, after air travel, the risk for venous thrombosis is increased between two- and four-fold. However, Suzanne Cannegieter, from Leiden University Medical Center, and colleagues point out that the absolute risk for venous thrombosis after air travel is unknown.

The team therefore studied 8755 employees from large international companies and organizations between 2000 and 2005, correlating travel records provided by the employers with the occurrence of symptomatic venous thrombosis. The researchers considered a flight of at least 4 hours long haul, and that exposure continues for a postflight period of 8 weeks.

The participants were followed-up for a total of 39,910 person-years, with employees exposed to a long-haul flight for a total of 6872 person-years. There were 53 thromboses during the follow-up period, of which 22 occurred within 8 weeks of a long-haul flight. The majority of flight-related thromboses occurred within 2 weeks of air travel.

The team calculates that the incidence rate of thrombosis among participants exposed to air travel was 3.2 per 1000 person-years, compared with a rate of 1.0 per 1000 person-years among those not exposed to air travel, giving an incidence rate ratio of 3.2. This was equivalent to a risk of one thrombosis event per 4656 long-haul flights.

The risk was increased by exposure to an increased number of flights within a short time period and with increasing duration of flights. The risk was also increased in participants aged under 30 years, women who used oral contraceptives, and those who were particularly short, tall, or overweight.

"The results of our study do not justify the use of potentially dangerous prophylaxis such as anticoagulant therapy for all long-haul air travellers, since this may do more harm than good," the researchers say. "However, for some subgroups of people with a highly increased risk, the risk-benefit ratio may favor the use of prophylactic measures."

The research is published in the journal PLoS Medicine.

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