NEW YORK (Reuters) --The risk of blood clots is mildly elevated by air travel lasting six hours or more, according to three reports in the Archives of Internal Medicine.
However, a much higher risk is seen in people with a propensity toward clotting, such as those who use birth control pills.
Long air trips greatly increase the risk of a blood clot traveling to the lungs, known as pulmonary embolism, but a person's chances of experiencing this serious problem is still quite low.
"Special precautions are probably not required for flights less than 6 hours, but may become appropriate when passengers with risk factors for (pulmonary embolism) embark on prolonged flights of six hours or longer," Dr. David Jimenez, with the Ramon y Cajal Hospital in Madrid, and his associates write.
In one study, Dr. Ida Martinelli and colleagues at the University of Milan, Italy, identified 210 patients who had experienced a clotting episode in the last 24 months. The travel histories of these patients were compared with similar subjects without clotting problems.
Recent air travel doubled the risk of clots, the authors note. The risk was tripled when considering only flights of eight hours' duration or longer.
Compared with individuals who did not fly and did not have a propensity toward clotting, the risk of clots was six times higher in patients with a clotting propensity, but 16 times higher in those with both risk factors.
Similarly, use of birth control pills increased the risk of clots fourfold, while the risk was 14 times higher among pill users who also flew.
Meanwhile, a team of researchers at the University Hospital of Dresden Medical School, Germany, led by Dr. Thomas Schwarz, conducted a study that included 964 individuals who were tested for clots before and within 48 hours after air flights lasting 8 hour of more. Similar examinations were conducted in 1213 non-traveling control subjects.
Clotting events were documented in 2.8 percent of passengers and 1 percent of controls. However, Schwartz' team notes, the elevated risk applied "only to individuals with preexisting permanent risk factors."
Jimenez's group in Madrid limited their study to international travelers in whom pulmonary embolism was diagnosed over a six-year period. Out of the nearly seven million passengers per year on international flights, only 16 were diagnosed with pulmonary embolism. The risk was elevated only among those whose flights lasted six hours or more.
Thus, "the incidence does not justify the social alarm created around air travel caused by the risk of developing (blood clots)," they write.
All three research groups concur with the conclusions of Dr. James E. Dalen in his editorial, that individuals at risk should walk around during extended travel and consider using compression stockings. The evidence suggests that those at high risk for clots may benefit from blood-thinning drugs.
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