Long-haul flights 'triple' risk of DVT

Your chances of getting DVT are tripled from taking long haul flights, newspapers reported today. A study has found that women, especially those taking the contraceptive pill, and people who are particularly tall, short and overweight are most at risk.

This research gives us a good estimate of how common venous thrombosis (DVT or pulmonary embolism) is after long haul flights. Although the study found that the relative risk of DVT is tripled, it also found that the absolute risk is still only one venous thrombosis for every 4,656 long haul flights.

It also makes clear that using drugs to prevent venous thrombosis may not be beneficial for all travellers. One author of the study is reported as saying, “The results of our study do not justify the use of potentially dangerous [treatment] such as anticoagulant therapy for all long haul air travellers, since this may do more harm than good.”

There are non-medical measures that people can take to reduce the risk of venous thrombosis, such as walking around and stretching during the flight.

Where did the story come from?

Saskia Kuipers and colleagues from Leiden University and medical centres in Holland and Switzerland carried out this research. The study was funded by the Netherlands Heart Foundation, the UK Government, and the European Commission. The study was published in the peer-reviewed medical journal: PLoS Medicine.

What kind of scientific study was this?

This was a cohort study that looked at the absolute risk of getting venous thrombosis (either deep vein thrombosis or pulmonary embolism) after flying.

Between 1998 and 2006, researchers enrolled 8,755 people who worked for organisations, which kept records of their employees’ business travel. The researchers used the company records to find out how many flights each person had been on, and how long the flights were. Flights of four hours or longer were classified as long-haul flights.

Participants answered questionnaires about whether they had other potential risk factors for venous thrombosis (such as being overweight or taking the contraceptive pill), and whether they had experienced venous thrombosis. Any cases of self-reported venous thrombosis were confirmed and dated using medical records, and researchers only counted the first instances of symptomatic venous thrombosis that were diagnosed using accepted methods.

Researchers identified the eight weeks following a long haul flight as the period when the participants were “exposed” to risk of thrombosis and calculated how common venous thrombosis was in these periods compared to periods of non-exposure.

Researchers also calculated the absolute risk of a venous thrombosis after a flight. They also looked at whether the risk increased with increasing numbers of flights, increasing length of flight, or in people who had different characteristics and risk factors for venous thrombosis.

What were the results of the study?

Overall, venous thrombosis was about three times more common in the eight weeks after a long haul flight than at other times. They found that this equated to an absolute risk of one venous thrombosis for every 4,656 long haul flights. The risk increased more with more flights, and with flights of longer durations. The increase in risk was greater in younger people (less than 30 years old), women who used the contraceptive pill, people who were overweight, and people who were shorter than 5ft 4in or taller than 6ft.

What interpretations did the researchers draw from these results?

The researchers concluded that the risk of venous thrombosis increases slightly after long haul flights overall, but that this risk does not justify the use of preventative measures, such as anticoagulants, for all long haul travellers. These measures carry their own risks that may outweigh any potential benefits. However, they also conclude that their study has identified some groups at particularly high risk, and that preventative measures may be useful in specific groups of people such as these. They suggest that large randomised controlled trials will be needed to confirm which groups will benefit most.

What does the NHS Knowledge Service make of this study?

This was a relatively large study, whose overall results give a reasonable estimate of the risk of having a venous thrombosis after a long haul flight. The important thing to note from this study is that the absolute risk of having an event is quite low, even if you do take long haul flights. Other factors that should be taken into account are:

• Only a small proportion (about a third) of the employees asked to take part in the study actually did take part. It is possible that the results of the people enrolled may not have been representative of the population as a whole.
• The study did not appear to have asked participants about flights taken for non-business purposes. If participants took long haul flights for non-business purposes and had venous thromboses afterwards these would have been incorrectly counted as happening outside of the “exposed” times.
• However unlikely, there is the chance that some participants might not remember if and when they had venous thrombosis (called recall bias). There is a greater chance that some of the participants might remember the event, but not know that it was a “venous thrombosis”. If this did occur, it would have affected the absolute risks found by the study.

It is reassuring that the study found that the absolute risk of DVT is low. However, as the researchers note, for some subgroups of people at an increased risk “the risk-benefit ratio may favour the use of prophylactic measures”. I.e. the reduced risk of getting a thrombosis outweighs the risks of taking anti-coagulants such as warfarin. Answering the question of how to balance the benefit of anti-clotting treatments (in reducing the rate of DVT) with any harm (e.g. avoiding their adverse effects) will require further studies.

Sir Muir Gray adds...

Although the evidence is not strong, and more research is undoubtedly needed, I take half an aspirin the morning of a flight and walk and do knee bends every hour while flying. However, if it’s a business trip, a better alternative is simply to send a video of my talk; better for my veins - better for the planet.