March 23 (HealthDay News) -- Children and siblings of people who develop blood clots in the veins may be more than twice as likely as those without a family history to develop the condition, Dutch researchers report.

This kind of clot, known as a venous thrombosis, can be potentially dangerous, because it can break off and travel to the lungs.

The study authors compared 1,605 venous thrombosis patients with a control group of 2,159 people who never had the condition. The study found that 505 (31.5 percent) of the venous thrombosis patients had at least one first-degree relative with a history of the condition, compared with 373 (17.3 percent) of those in the control group.

The Leiden University Medical Center researchers said family history was associated with a more than twofold increased risk of venous thrombosis. The risk was even greater if the relative developed blood clots at a young age and as much as four times higher if more than one relative had venous thrombosis.

There wasn't a strong link between family history and known genetic risk factors, which suggests that there may be unknown genetic risk factors or that venous thrombosis may affect family members due to household factors, the researchers said.

"Both in those with and without genetic or environmental risk factors, family history remained associated with venous thrombosis. The risk increased with the number of factors identified; for those with a genetic and environmental risk factor and a positive family history, the risk was about 64-fold higher than for those with no known risk factor and a negative family history," wrote Irene D. Bezemer and colleagues.

Family history may be more useful for risk assessment than laboratory tests that identify genetic or physiological risk factors, the researchers suggested.

The study is published in the March 23 issue of the Archives of Internal Medicine.

SOURCE: JAMA/Archives journals, news release, March 23, 2009