Travel may up risk of clots

Long-distance travelling triples the risk of potentially dangerous blood clots, and the risk goes up the longer you are travelling, according to new research. But it's important to keep this new finding in perspective, as for most people the risk is still very low.

What do we know already?
Normally, your blood only forms clots if you injure yourself, to stop the bleeding. But clots can also form inside the deep veins of the body, usually in one of the legs. The leg may swell up and be painful. If part of the clot breaks up and travels through the vein to the lungs, it can be fatal.

A blood clot in a vein is called a venous thromboembolism (or VTE). It is more likely to happen if you are overweight, if you're pregnant or taking the contraceptive pill, or if you're unable to walk about because of illness or injury. There has already been lots of research looking at whether long-distance travel, particularly air travel, increases your risk of a blood clot forming. So far the results haven't been conclusive. Researchers have now looked again at all the studies to see if they can draw any conclusions.

What does the new study say?
The researchers say that if you pool all the data from the studies, it shows that travel doubles the risk of blood clots. They say the studies that didn't show an increased risk had problems that made them unreliable, and that if you take out the figures from these less reliable studies, travel almost triples the risk of blood clots.

They also calculated that the longer your journey takes, the higher your risk becomes. For people travelling by air, the risk goes up by about a quarter for each additional 2 hours that the journey lasts.

How reliable are the findings? We can be fairly sure from this study that travelling long distances is linked to a higher risk of blood clots, and that the longer you travel the greater the risk. From what we know about the way the blood circulates, the risk is probably due to sitting still for a long time.

The research was done by doctors from the Harvard School of Public Health, Harvard University, Boston, USA. It was published in the medical journal Annals of Internal Medicine.

What does this mean for me?
Phrases like 'doubles your risk' and 'triples your risk' sound frightening. But it's important to know what your risk is to start with. This particular study doesn't tell us the risk for most people. But another study found the risk for healthy people was quite low. For every 4,600 healthy people taking a flight, one would get a blood clot. So tripling the risk would mean 3 people in 4,600 get a blood clot, which is still very low.

However, the new information might be more important for people at higher risk of getting a blood clot (for example, pregnant women or people who are very overweight). It tells us that the risk goes up when the journey is longer, so if you're a pregnant woman planning to fly to Australia, you might want to talk to your doctor about precautions to help avoid blood clots.

What should I do now?
If you think you may be at risk of blood clots and are planning a flight or other long journey, talk to your GP, practice nurse, or pharmacist about what precautions you need to take to avoid getting a blood clot. Some people may be advised to take medication or wear special elastic stockings to help keep the blood flowing. Although there's not much research on other measures, most specialists recommend trying to exercise your legs when seated, and to avoid sitting still for long periods, even if you're not at particular risk.

The Guardian