MONDAY, Aug. 8 (HealthDay News) -- People who have two or more siblings who have suffered blood clots in deep veins such as those in the legs and pelvis -- a disease known as venous thromboembolism (VTE) -- have a relative risk 50 times higher for developing such clots themselves, Swedish researchers report.

Individuals with only one sibling with VTE are two times as likely to suffer the dangerous blood clots.

This is the first study in a large population to show that the risk for VTE runs in families, the researchers say. VTE causes blood clots called deep vein thrombosis, which, if they break loose, can travel to the heart, lungs or brain and, if untreated, tend to be fatal.

"We found genetic factors are important in the risk for VTE," said lead researcher Dr. Bengt Zoller, an associate professor, at the Center for Primary Health Care Research at Lund University in Malmo.

"A sibling history of VTE is an important risk factor for VTE," he said. However, Zoller pointed out that most people who develop a VTE don't have a family history of the condition.

While the relative risk is very high, the absolute risk is much lower. In the general population, the absolute risk for VTE is 3 in 10,000 each year and for those whose family history puts them at high risk it is 15 in 10,000, each year, Zoller said.

The report is published in the Aug. 8 online edition and the Aug. 30 print issue of Circulation.

For the study, Zoller's team collected data on 45,362 people who were hospitalized with VTE. Among these people, 2,393 had a brother or sister who also had a history of the condition.

The researchers found that for those aged 10 to 19 years, there was a five times greater risk of VTE if they had a sibling who had had a VTE, compared with those who didn't have this family history. For older patients, those 60 to 69, the risk was twice as high.

Taking into account age differences between siblings, Zoller's group determined that no environmental factor played a role in the increased risk of VTE.

In addition, while VTE was found in both men and women, the rate was higher among women, especially those 10 to 40 years old. After 50, however, the risk was higher in men than women, the researchers noted.

Risk factors for VTE include surgery, heart failure, smoking, obesity, cancer, long periods of inactivity (such as when driving or flying), sitting or lying in bed, fractures in the legs or hip and taking birth control pills.

Modifying these risk factors can lower your risk for VTE, Zoller said. After heart attack and stroke, VTE is the most common cardiovascular illness and affects one in 1,000 people each year, the researchers said.

Commenting on the study, Dr. Jack Ansell, chairman of the department of medicine at Lenox Hill Hospital in New York City, said that "the study confirms a lot of what we know, but it also provides impetus to greater investigation of genetic and non-genetic factors."

However, Ansell noted that genetics is not the whole story. "There are many individuals with inherited predisposition that never have a problem, so it's not an all or none phenomenon," he said.

Conversely, if you don't have a family history of VTE, that doesn't mean that you will not have one, Ansell said. "There are clearly people who have no risk for a VTE who end up with an event due to acquired factors like an accident or cancer," he said.

This study should not cause people to panic, as the risk for having a VTE is low regardless of family history, Ansell said.

However, he added that people with a family history of VTE should let their doctor know before undergoing any surgery. This will allow the doctor to take preventive measures to prevent the risk of clotting.

More information

For more information on deep vein thrombosis, visit the U.S. National Library of Medicine.