Dutch researchers have found that the risk of blood clots rises more than two-fold in children and siblings of people with venous thrombosis or blood clots in the veins than in those sans such a family history.
Venous thrombosis typically begins in leg veins, although the clot may subsequently break off and travel to the lungs.
"Because universal screening is not cost-effective, research efforts are focused on selection criteria that may be used to increase the chance of finding a genetic risk factor," the authors write.
"Family history is an evident candidate," they added.
To reach the conclusion, Irene D. Bezemer, M.Sc., and colleagues at Leiden University Medical Center, Leiden, the Netherlands, collected blood samples and information about family history and environmental risk factors from 1,605 patients who had experienced their first clot between 1999 and 2004.
Their data was compared with that of 2,159 control participants who were the same sex and age but had not had venous thrombosis.
Among patients with venous thrombosis, 505 (31.5 percent) had at least one first-degree relative with a history of the condition, compared with 373 controls (17.3 percent). A positive family history was associated with a more than two-fold increase in the risk of venous thrombosis; the risk was increased further if the relative developed clots at a younger age and as much as quadrupled if more than one relative was affected.
Family history did not correspond well with known genetic risk factors, suggesting that there may be unknown genetic risk factors or that venous thrombosis may cluster in a family due to characteristics of the shared household, the authors note.
"Both in those with and without genetic or environmental risk factors, family history remained associated with venous thrombosis," the authors write.
"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," they added.
The relative risk associated with family history was similar to that associated with a genetic risk factor.
The study has been published in the March 23 issue of Archives of Internal Medicine, one of the JAMA/Archives journals.