updated 7/28/2006 11:18:04 AM ET 2006-07-28T15:18:04

Among people who have had blood clots, men are twice as likely as women to have them again after finishing treatment, according to an analysis of several studies.

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"Gender may need to be considered when deciding how long patients should be treated with blood thinners," said Dr. Simon McRae, principal author of a paper published Friday in the medical journal Lancet.

McRae is a doctor with the department of hematology and oncology at Queen Elizabeth hospital in Woodville, Australia. He and his colleagues analyzed 15 studies in a review that considered approximately 5,400 people who had discontinued medication three to six months after having a blood clot.

Blood clots, or thrombosis, are thought to affect 1 in every 1,000 people. The clots develop in veins of the legs, which then get dislodged and can travel to the heart or lungs, potentially causing cardiac or pulmonary failure.

The condition has also been called "economy class syndrome," since remaining immobile for long stretches of time, for example during long-haul flights, is a known risk factor.

The Lancet study only concerns people with a past record of blood clots.

Statistically higher risk
There is no discernible difference in the risk of a first blood clot between men and women. When it comes to recurrent thrombosis, however, McRae concludes that men appear to be statistically at higher risk.

McRae said that the discrepancy is significant enough that physicians should take gender into account when treating patients.

"The real potential of this study is that it will help doctors determine how long certain patients should remain on blood thinners," he said.

Researchers are still trying to determine why men are more susceptible to recurrent blood clots than women. They believe physiological, genetic or hormonal variations might play a role.

"We know that diseases manifest in different ways between men and women," said Dr. Sidney Smith, director of the center for cardiovascular science at the University of North Carolina. "The next step is to design a prospective study so that we can see if these observed differences are actually real."

In an accompanying commentary in the Lancet, scientists in Italy conclude that "it is still too early to rely on patients' sex when determining the length" of treatment on blood-thinners.

"There is a lot of data available that could be examined in more detail," said Dr. Sania Nishtar, founder of Heartfile, a health-policy think tank in Pakistan, and frequent adviser to the World Health organization. "The Lancet study may not have found any definitive answers, but what it has done is to flag important research questions," said Nishtar.

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