Background: A positive family history of venous thrombosis may reflect the presence of genetic risk factors. Once a risk factor has been identified, it is not known whether family history is of additional value in predicting an individual's risk. We studied the contribution of family history to the risk of venous thrombosis in relation to known risk factors.
Methods: In the Multiple Environmental and Genetic Assessment of risk factors for venous thrombosis, a population-based case-control study, we collected blood samples and information about family history and environmental triggers from 1605 patients with a first venous thrombosis and 2159 control subjects.
Results: A total of 505 patients (31.5%) and 373 controls (17.3%) reported having 1 or more first-degree relatives with a history of venous thrombosis. A positive family history increased the risk of venous thrombosis more than 2-fold (odds ratio [95% confidence interval], 2.2 [1.9-2.6]) and up to 4-fold (3.9 [2.7-5.7]) when more than 1 relative was affected. Family history corresponded poorly with known genetic risk factors. 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.
Conclusions: Family history is a risk indicator for a first venous thrombosis, regardless of the other risk factors identified. In clinical practice, family history may be more useful for risk assessment than thrombophilia testing.