The Most Common Inherited Clotting Mutation
Factor V Leiden is a genetic mutation that makes Factor V resistant to being inactivated by Protein C. Normally, Protein C puts the brakes on clotting by degrading Factor V. With this mutation, Factor V ignores the brake signal and keeps promoting clotting. It's the most common inherited thrombophilia—about 5% of Caucasians carry it.
What is Factor V Leiden?
Factor V Leiden is a point mutation (G1691A) in the Factor V gene causing an amino acid substitution (R506Q) at the Protein C cleavage site. This makes Factor Va resistant to inactivation by activated Protein C (APC resistance). Most common inherited thrombophilia. Autosomal dominant.
↑ What High Factor V Leiden Means
Heterozygous (one copy): 5-7x increased clot risk. Homozygous (two copies): 50-80x increased risk. The mutation doesn't cause clots on its own in most cases—it increases risk, especially when combined with other factors (OCP, surgery, immobility).
Common symptoms:
Most carriers never have a clot · DVT: leg pain, swelling, redness · PE: sudden dyspnea, chest pain · Recurrent pregnancy loss (associated)
↓ What Low Factor V Leiden Means
N/A—this is a genetic yes/no test.
Common symptoms:
N/A
Why It Matters
When normal:
Explains unexplained DVT/PE, especially at young age
Guides anticoagulation duration decisions
Helps with family counseling
Informs decisions about OCP use
Risks if abnormal:
Heterozygous: 5-7x VTE risk
Homozygous: 50-80x VTE risk
Synergistic risk with OCP (35x in heterozygous women on OCP)
Does NOT increase arterial event (MI, stroke) risk
What Can Cause Abnormal Levels?
Inherited Genetic Mutation
100% likelyAutosomal dominant. 5% Caucasian prevalence. Much rarer in African and Asian populations.
This is a fixed genetic trait—not modifiable
Focus is on managing additional risk factors to prevent clotting events.
What You Can Do
Avoid additional clot risk factors: no smoking, stay active, adequate hydration
Impact: Reduces additive risk \u00B7 Timeline: Lifelong
Move during long travel (walk every 1-2 hours on flights)
Impact: Prevents stasis-related clots \u00B7 Timeline: Ongoing
Discuss OCP alternatives with gynecologist (estrogen + FVL = high risk)
Impact: OCP increases risk 35x in heterozygotes \u00B7 Timeline: Important discussion
If lifestyle changes aren't enough:
Prophylactic anticoagulation for high-risk situations (surgery, hospitalization)
Impact: Prevents provoked DVT/PE \u00B7 Timeline: Perioperative
Family screening
Impact: 50% chance children carry mutation \u00B7 Timeline: One-time
Recommended retest: One-time genetic test—does not change
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