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 say.

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

Researchers in Sweden used a nationwide hospitalization registry to explore the influence of family risk, as determined by sibling history, on the dangerous blood clots.

The study is the first to show a direct correlation between the blood clots and family risk in a nationwide setting.

"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. 

About one in 1,000 Americans develops VTEs each year, and about a third of these cases involve pulmonary embolism, according to the American Heart Association.

Established risk factors for VTE include immobilization, surgery, trauma, pregnancy, malignancy and oral contraceptive use.

It has long been recognized that family history is an important risk factor for the deadly blood clots, but much remains to be learned about the role genetics play in the condition, Zoller tells WebMD.

Family influence played a significant part in risk for both males and females, with about 5 percent of hospitalized patients having a sibling who had also been hospitalized with VTE.

When one sibling had a history of VTE, a person's risk was double that of people with no family history of the blood clots.

When two siblings had been hospitalized with the blood clots, however, risk increased 50- to 60-fold.

Among the other findings:

  • Having a sibling hospitalized with a blood clot was associated with a nearly fivefold increase in risk among children and teens between the ages of 10 and 19.
  • A similar history was associated with a doubling of risk among people between the ages of 60 and 69.
  • Females had a slightly higher risk for the blood clots compared to males from age 10 until the age of 40, and men had a slightly higher risk after the age of 50.

The study appears online in the American Heart Association journal Circulation.

This study should not cause people to panic, as the risk for having a VTE is low regardless of family history, Dr. Jack Ansell, chairman of the department of medicine at Lenox Hill Hospital in New York City, told U.S. News and World Report.

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.