Travel more than doubles risk of blood clots: study

By Amy Norton

NEW YORK (Reuters Health) - A study published Monday strengthens the evidence that long-distance travel can lead to potentially fatal blood clots in some people -- showing that the risk grows in tandem with the length of the trip.

In an analysis of 14 previous studies, researchers found that, in general, travel was associated with a nearly three-fold increase in the risk of venous thromboembolism (VTE) -- blood clots that form in the veins, often in the legs.

If such a clot dislodges and travels to the lungs, it can cause a potentially fatal condition called pulmonary embolism.

Several high-profile deaths have brought attention to the risk of VTE among travelers, particularly those on long-haul flights. Experts think a combination of factors -- including dehydration and hours of sitting in cramped conditions -- explains why some people develop blood clots.

However, not all studies have found a clear link between travel and VTE. To look at the discrepancy, the researchers who conducted the current review, published in the Annals of Internal Medicine, analyzed 14 studies involving more than 4,000 cases of VTE.

Some of the studies compared VTE patients with a "control" group of people who had been referred for possible VTE symptoms, but were found to not have a clot -- a comparison that carries the risk of bias because the control group likely has a higher-than-average risk of blood clots.

In other studies, the control group consisted of healthy people from the general population -- which are more likely to capture the true VTE risk associated with travel, explained lead researcher Dr. Divay Chandra of the Harvard School of Public Health in Boston.

When the researchers looked only at those studies, they found that travelers had a nearly three-fold higher risk of blood clots than non-travelers.