Prothrombin gene mutation (Factor II G20210A)
A genetic test for a common inherited change in the prothrombin (Factor II) gene that makes the blood slightly more prone to clotting. It is one of the two most common inherited clotting tendencies (with Factor V Leiden) and is checked when working out why someone had, or is at risk of, a blood clot.
What it measures
Prothrombin (Factor II) is a key protein in the clotting cascade. A specific single-letter change in its gene, called G20210A, causes the body to make slightly more prothrombin than normal, which tips the balance a little towards clotting — an inherited condition called thrombophilia. The test is a genetic one, reporting whether a person carries no copy, one copy (heterozygous, the common situation) or two copies (homozygous, rarer) of the change. Along with Factor V Leiden, it is one of the two most common inherited clotting tendencies, especially in people of European descent. Carrying it modestly increases the lifetime risk of a venous clot — a deep vein thrombosis in the leg or a pulmonary embolus in the lung — and the risk is higher when combined with other factors such as pregnancy, the contraceptive pill, surgery, immobility or another clotting gene. It does not cause clots in most carriers, and is not tested for routinely; it is checked in specific situations, such as an unexplained or early clot, or a strong family history, to help understand risk and guide decisions.
What a high value can mean
- Heterozygous (one copy) — the common carrier state; a modest increase in clotting risk.
- Homozygous (two copies) — rarer; a higher risk.
- Combined with Factor V Leiden or other factors — risks add up (pregnancy, the pill, surgery, immobility).
- Note: many carriers never have a clot; the result describes risk, not a certainty.
High Prothrombin gene mutation (Factor II G20210A): full guide
What a low value can mean
- Not present (no copy) — the common, reassuring result; this particular inherited clotting tendency is absent.
Low Prothrombin gene mutation (Factor II G20210A): full guide
When to discuss with a doctor
The prothrombin gene mutation is not a routine screening test; it is checked in specific situations — an unexplained blood clot, a clot at a young age or an unusual site, recurrent clots, or a strong family history — usually alongside Factor V Leiden and other thrombophilia tests. A positive result does not mean a clot is inevitable: most carriers never have one, and the finding is used to understand risk and to inform decisions around higher-risk times such as pregnancy, hormone use, surgery or long travel. Whether it changes management (for example, the choice of contraception or the need for clot prevention at certain times) is an individual decision made with your doctor. Mediora.AI shows the result alongside related clotting markers for context; the interpretation and any preventive plan belong with your doctor.