Aug. 16, 2011—People whose siblings have had dangerous blood clots in the legs or pelvis have an increased risk of developing clots themselves, according to a study published in Circulation: Journal of the American Heart Association.

The Swedish study included 45,362 cases of people hospitalized for venous thromboembolism (VTE).

VTE is the third most common cardiovascular disease, after heart attack and stroke, according to researchers. VTE consists of deep vein thrombosis (DVT)—which usually involves the formation of blood clots in the deep veins of the legs or pelvis—along with a complication of DVT known as pulmonary embolism, in which a blood clot breaks off and travels to the lungs, where it becomes lodged in the pulmonary arteries.

Of the cases included in the study, 5.3 percent had a sibling history of VTE. Researchers found that:

• People ages 60 to 69 had twice the VTE risk if a sibling had had the disease, compared to those whose siblings didn't have blood clots.
• For those ages 10 to 19, the risk was five times greater for those with a sibling history of VTE compared to those whose siblings didn't have clots.
• If a person had two or more siblings with a history of VTE, their own risk was 50 times greater than someone whose siblings didn't have clots.

“Hereditary factors—as determined by sibling history—are significant in determining the risk of venous thromboembolism in men and women between the ages of 10 and 69,” said Bengt Zöller, MD, PhD, senior study author and associate professor at the Center for Primary Health Care Research, Lund University in Malmö, Sweden. “More importantly, in a fraction of the families we studied, the risk for venous thromboembolism was unusually high—50 to 60 times higher than those families who were not at risk, thus suggesting a strong genetic risk factor.”

More studies are needed to show how genetics may influence the risk of blood clots, Dr. Zöller said.

Learn more about DVT here.