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Blood Clots in People with the Factor V Leiden Mutation
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What is the problem and what is known about it so far?
Blood clots occur commonly in deep veins in the legs. Pieces of these clots, known as emboli, can break off and travel through the bloodstream, blocking arteries in the lungs. These blood clots and emboli can cause serious symptoms and death. Surgery, pregnancy, trauma, hormone therapy, and inherited blood disorders increase risks for blood clots and emboli. Factor V Leiden is one of these inherited disorders. Although factor V Leiden is a common genetic defect, the actual incidence of clots and emboli among people with the defect is unknown.
Why did the researchers do this particular study?
To find out how often blood clots and emboli occur in people with the factor V Leiden mutation.
Who was studied?
470 relatives of patients with factor V Leiden mutation and known blood clots or emboli. All of the relatives also had factor V Leiden mutation, but none had histories of blood clots or emboli.
How was the study done?
Patients with blood clots or emboli and factor V Leiden mutation were identified from three hospitals in the Netherlands. The researchers contacted parents, siblings, and children of the patients. These relatives were asked if they had ever had blood clots in their legs or lungs, and their blood was tested for factor V Leiden mutation. Relatives with factor V Leiden mutation and no history of blood clots were taught how to watch for symptoms and signs of clots. Every 6 months for about 3 years, they were asked about blood clots and related risk situations, such as surgery, pregnancy, or taking hormone pills. Medical charts of those who reported blood clots were reviewed to confirm diagnoses.
What did the researchers find?
Nine relatives had blood clots in the deep veins of the legs or emboli to their lungs. Four instances occurred in relatives with no risk factors, and five occurred in relatives with increased risk for blood clots (1 after surgery, 1 during estrogen replacement therapy, and 3 during use of birth control pills). The overall occurrence rate of deep vein thrombosis or lung emboli was 0.58% per year. The rate was 1.8% per year of birth control pill use; 2.9% per year of hormone replacement therapy use; and 3.5% per episode of surgery, trauma, or prolonged immobilization.
What were the limitations of the study?
Precise estimates of rates in people at higher risk, such as those undergoing surgery or taking birth control pills, were not possible because few people with these situations were studied.
What are the implications of the study?
The yearly occurrence rate of deep vein clots or lung emboli among people with factor V Leiden mutation and no history of clots or emboli seems low (about 6 events per 1000 people with the mutation). Although this rate is five to six times higher than that seen in the general population, the authors concluded that the rate is too low to justify routine genetic screening of family members of patients with clots and factor V Leiden mutation.
Article and Author Information
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The summary below is from the full report titled “A Prospective Study of Asymptomatic Carriers of the Factor V Leiden Mutation To Determine the Incidence of Venous Thromboembolism.” It is in the 4 September 2001 issue of Annals of Internal Medicine (volume 135, pages 322-327). The authors are S Middeldorp, JR Meinardi, MMW Koopman, ECM van Pampus, K Hamulyák, J van der Meer, MH Prins, and HR Büller.
- Copyright ©2004 by the American College of Physicians
