Aug. 8, 2011 -- People with more than one sibling who have had potentially life-threatening blood clots have a 50-fold increase in risk for the condition, a new study shows.
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.
Blood Clots Are Silent Killers
Known medically as deep vein thrombosis (DVT), blood clots that form in the deep veins of the legs or pelvis can become deadly when they travel to the lungs and block pulmonary arteries. DVT and its complication, pulmonary embolism, are known collectively as venous thromboembolisms (VTE).
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 is still not known about the role that genetics play in the condition, Bengt Zoller, MD, PhD of Malmo, Sweden's Lund University tells WebMD.
"There appear to be a large number of genetic risk factors for this condition that have not yet been identified," he says.
When Zoller and colleagues examined the impact of heredity on VTE by age and gender using the Swedish hospital registry data, they identified more than 45,000 hospitalizations for the condition between 1987 and 2007.
Patients ranged in age from 0 to 75, with the average age of hospitalization being 51 for men and 46 for women.
Family influence played a significant part in risk for both males and females, with about 5% 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.
Genetic Risk Factors for VTE
Five major genetic risk factors associated with an increased risk for VTE have been identified. But doctors do not routinely test for them, so most people do not know if they carry one or more of the genetic risk factors.
This was the case with NBC News correspondent David Bloom, who died of a blood clot that started in his legs and traveled to his lungs in the early days of the Iraq war after spending days immobilized in a tank while embedded with U.S. troops.
Although Bloom did not know it, he had inherited a blood clotting disorder that increased his risk for the dangerous blood clots, along with other risk factors, his wife, Melanie, later said.
Philadelphia cardiologist Don LaVan, MD, who is a spokesman for the American Heart Association, says the study confirms that asking patients about their family history of VTE is important for assessing risk.