Factor V Leiden
The commonest inherited cause of a tendency to form abnormal blood clots. It is a genetic change in clotting factor V that makes it resistant to being switched off, so clotting is harder to stop. The result is reported as present or absent, not a number, and most carriers never have a clot.
What it measures
Factor V Leiden is a specific inherited mutation in the gene for clotting factor V, one of the proteins in the cascade that forms a blood clot. Normally, a natural brake called activated protein C switches factor V off once enough clot has formed, keeping clotting in check. The Leiden mutation makes factor V resistant to this brake — a state called activated protein C resistance — so clotting is turned off less efficiently and blood is slightly more prone to clotting than normal. It is the commonest inherited thrombophilia (clotting tendency), especially in people of European descent. Testing is done either functionally (an activated protein C resistance assay) or as a direct genetic test, and the result is qualitative: absent (normal), heterozygous (one copy inherited, a modest increase in risk), or homozygous (two copies, a larger increase). Crucially, most people who carry factor V Leiden never develop a clot; it raises the risk rather than guaranteeing a problem, and that risk is amplified by other factors such as surgery, pregnancy, the combined contraceptive pill, immobility and long-haul travel. It is usually tested after an unprovoked or unusual clot, or a strong family history.
What a high value can mean
- Heterozygous factor V Leiden (one copy) — the common form; a modest increase in the lifetime risk of venous clots (DVT, pulmonary embolism).
- Homozygous factor V Leiden (two copies) — less common; a larger increase in clotting risk.
- Risk multiplied by other factors — pregnancy, the combined pill or HRT, surgery, immobility, long flights.
- A clue in someone with an unprovoked or recurrent clot — helps explain why it happened.
What a low value can mean
- Absent (normal) — no factor V Leiden mutation; the expected result in most people.
- A normal activated protein C resistance test — clotting is switched off normally.
When to discuss with a doctor
Factor V Leiden testing is done in specific situations rather than as a general screen — typically after an unprovoked or unusual blood clot (a DVT or pulmonary embolism), a clot at a young age, recurrent clots, or a strong family history of clotting. A positive result does not mean a clot is inevitable: most carriers never have one, and the practical response is usually to be aware of higher-risk situations (surgery, long flights, pregnancy, the contraceptive pill) and take sensible precautions, rather than lifelong blood thinners. Decisions about testing, contraception, pregnancy care and any treatment are individualised and belong with a haematologist. Mediora.AI can surface a factor V Leiden result alongside the other thrombophilia markers so the clotting-risk picture is read together; interpreting what it means for you belongs with your doctor.