Thrombophilia, Factor V Leiden
Definition
The Leiden variant in the F5 gene disrupts the natural brake of the clotting system and increases the risk of venous thromboembolism (VTE).
Risk levels:
| Genotype | Relative risk of VTE | Note |
|---|---|---|
| Heterozygous | ~3 to 8-fold increase | Annual incidence of first VTE ~0.5%. Adjusted HR: 2.7 (95% CI 1.8 to 3.8) |
| Homozygous | ~9 to 80-fold increase | Adjusted HR: 18 (95% CI 4.1 to 41). Thrombosis tends to occur at a younger age |
Frequency in the population: Heterozygosity for the Factor V Leiden variant is seen in 3 to 8% of the general US and European populations. The frequency of homozygosity is about 1:5,000.
Results / Clinical significance: The most important limitation of the test is that it usually does not change treatment:
For asymptomatic heterozygous individuals, long-term prophylactic anticoagulation is not routinely recommended, because the bleeding risk exceeds the thrombosis risk.
The test mainly helps with decision-making in high-risk situations:
- Pregnancy
- Use of oral contraceptives (birth control pills)
- Major surgery, prolonged immobility
What the patient should know: A positive Factor V Leiden result does not mean "you will be on blood thinners for life." Most carriers never develop a clot.
Inheritance
Autosomal dominant