Prothrombin G20210A: the other inherited clotting variant

Genetics explainer · Updated June 2026

What prothrombin G20210A does

Prothrombin — also called Factor II — is a protein your body converts into thrombin, the enzyme that finishes the job of forming a clot. The G20210A variant is a single-letter change in the F2 gene. Rather than altering the protein itself, it sits in a region that affects how much prothrombin gets made, and carriers tend to have slightly higher prothrombin levels in the blood. More clotting factor available means clotting is a little easier to start and a little harder to keep in check.

The conditions it is associated with are the same family as Factor V Leiden: venous thromboembolism (VTE), mainly deep vein thrombosis (DVT) in the legs and pulmonary embolism (PE) in the lungs.

How much it actually raises risk

This is the part worth keeping in proportion. Prothrombin G20210A is generally considered a modest risk factor.

Two things keep this grounded. First, relative risk is not absolute risk: a two-to-threefold increase on a small baseline is still a small number, and the vast majority of carriers never have a clot. Second, clotting risk is rarely about one gene — it climbs around specific triggers and falls again when they pass.

The contexts that matter

The variant matters most when it overlaps with a situation that raises clotting risk on its own:

• Surgery and hospital stays — major operations and the immobility that follows are classic triggers, which is exactly why surgical teams ask about clotting history.
• Pregnancy and the weeks after birth — pregnancy naturally shifts the body toward clotting, and a known variant is useful context for an obstetric team.
• Estrogen-containing medication — combined hormonal contraceptives and some hormone therapies add their own clotting risk on top of any inherited factor.
• Long immobility — extended bed rest or very long-haul travel.

None of these means a carrier must avoid surgery, pregnancy, or travel. It means the variant is a useful piece of information to put on the table so a clinician can weigh the whole picture.

How it relates to Factor V Leiden

These two are siblings, not twins. They act on different proteins in the same clotting cascade and are inherited independently — you can carry one, both, or neither. Factor V Leiden makes Factor V resistant to being switched off; prothrombin G20210A tends to raise prothrombin levels. Because the effects stack, carrying both variants pushes risk higher than either alone. That stacking is one reason a clinician looks at your full history and any other markers rather than reacting to a single line in a raw-data file. If you have not read it yet, our companion explainer on Factor V Leiden (rs6025) covers the other half of the pair.

What to do with the result

If prothrombin G20210A flags as a carrier result — or if you have a personal or family history of clots regardless of what your file says — the right next step is a clinician and, where appropriate, a confirmatory clinical test, not a self-assessment. Consumer genotyping arrays can occasionally miscall a single position, and clotting risk is a medical judgment that weighs your history, other risk factors, and the situations ahead of you. Knowing your status is genuinely useful information to bring to that conversation; acting on it is a job for your doctor.

For the full genotype breakdown, see prothrombin G20210A (F2) in our gene library. To understand the other common inherited clotting variant, read Factor V Leiden in your raw data, or browse the rest of the Quanome blog.

Frequently asked questions

What is prothrombin G20210A?

It is a common inherited variant in the F2 gene, which codes for prothrombin (also called Factor II), a protein in the blood-clotting cascade. The variant tends to raise prothrombin levels slightly, which nudges the balance toward clotting. It is one of the two most common inherited clotting variants, alongside Factor V Leiden.

How much does it raise clotting risk?

For most carriers the increase is modest. Having one copy is associated with a roughly 2–3x higher relative risk of venous clots compared with non-carriers. But relative risk is not the same as absolute risk — the great majority of carriers never have a clot. Risk rises mainly around triggers like major surgery, long immobility, pregnancy, and estrogen-containing medication.

How is it different from Factor V Leiden?

They affect different proteins in the same clotting cascade and are inherited independently. Factor V Leiden makes Factor V resistant to being switched off; prothrombin G20210A tends to raise prothrombin levels. Carrying both is possible and pushes risk higher, which is one reason a clinician looks at the whole picture rather than a single marker.

Is a raw-data result a diagnosis?

No. Consumer raw data is a starting point, not a clinical test. A single-marker readout can be misread, and clotting risk is a medical matter that depends on far more than one gene. If this variant flags in your file, or you have a personal or family history of clots, see a clinician for confirmatory testing and advice.