Factor V Leiden

Most of what this result changes is ordinary. Whether you start a combined birth control pill. Whether you ask for clot prevention around a knee replacement, a long flight, or a pregnancy, and how hard you push for it.

Roughly one in twenty people of European ancestry carries a single-letter change in one clotting gene that makes clots form more easily than they should. No standard blood panel looks for it, and normal clotting times won't reveal it. You have to order the test.

What the Variant Changes

Factor V Leiden is a one-letter swap in the F5 gene, written c.1601G>A or Arg534Gln. Factor V itself is a clotting protein your liver makes, also stored inside platelets. Once it's switched on, it acts as an accelerator for clot formation.

The brake on that accelerator is a protein called activated protein C, which cuts factor V in a few places to shut it down. The Leiden variant removes one of those cut sites. The switched-on protein then lingers in the blood roughly ten times longer than it should, and your blood keeps building clots more readily than it otherwise would.

So this is a quality problem, not a quantity problem. You have the normal amount of the protein. It just doesn't turn off on schedule, and you carry one copy, two copies, or neither.

Clots in the Legs and Lungs

People get this wrong in both directions. Carrying one copy multiplies your relative risk of a clot several times over, and still, only about 10% of carriers ever develop an abnormal clot. Clots in the deep veins of the leg and clots that travel to the lung are the main events, though unusual sites show up too: carriers have higher rates of clots in the veins of the brain, in the arm, and in the veins just under the skin.

Sources: Copenhagen City Heart Study (Juul et al.); Malmö Diet and Cancer Study (Zöller et al.); Segal et al. systematic review.

A multiplied risk applied to a small baseline is still a modest number for most people with one copy. By age 65, about 6.3% of carriers have had a clot, against 5.2% of non-carriers. The annual risk of a first clot with one copy stays well under 1%, which is why the variant on its own almost never justifies blood thinners in someone who has never clotted.

One Copy or Two

The gap between one copy and two isn't a matter of degree. One copy runs about 2.9 clot events per 1,000 person-years. Two copies runs closer to 15, several times higher again, and the risk accumulated over a lifetime climbs with it.

Two copies also shows up in lifespan data, which is rare for a variant this common. In a population-wide Icelandic genetic study, people carrying two copies lived about 4.5 years less on average than non-carriers, alongside roughly 7.6 times the odds of a venous clot.

Circumstances stack on top of genotype, and they stack hard. Among Danish adults over 60 who smoked and had a BMI above 30, ten-year clot risk was about 10% with one copy and 51% with two. Carrying this variant alongside one copy of prothrombin G20210A, the second most common inherited clotting variant, lands you in a similarly high bracket.

Pregnancy, the Pill, and Hormone Therapy

This is where knowing your genotype earns its keep, because it changes decisions you make in advance rather than after something goes wrong. In pregnancy, women with two copies face around 34 times the clot risk of non-carriers. A woman carrying one copy of this variant plus one copy of prothrombin G20210A has roughly a 4% to 5% chance of a clot during pregnancy or the weeks after even with no prior clot, which is the level at which obstetric guidance starts weighing preventive blood thinners.

Estrogen pushes in the same direction, and the two multiply rather than add. Contraception guidance places combined estrogen-containing pills, patches, and rings in the highest-risk category for anyone with a known clotting variant of this kind, with progestin-only and non-hormonal methods used instead. Nobody recommends screening every woman before she starts the pill, so the question only comes up if you already know your status.

Pregnancy loss is the other association. Carriers have roughly 2 to 3 times the risk of repeated miscarriage or of losing a pregnancy in the second or third trimester. Preeclampsia, restricted fetal growth, and placental abruption have been reported in carriers, but those links aren't confirmed.

Repeat Clots Are a Different Question

The variant predicts your first clot much better than your second, which catches most people off guard. Pooled across studies, people with one copy had about 1.6 times the odds of a repeat clot, and people with two copies about 2.65 times. In one study of people whose clot came out of nowhere, recurrence curves for carriers and non-carriers were close to identical after stopping anticoagulation.

That isn't a contradiction once you see what each number measures. The genotype describes a standing tendency, which is useful information when nothing else about you says clot-prone. Once you've actually had a clot, the clot itself is the stronger predictor, and it already contains most of what the gene was telling you. This is why guidelines set the length of anticoagulation mainly by whether the clot had a trigger, not by genotype alone.

Heart Attack and Stroke

This is a vein problem, not an artery problem, and the largest study on the question is blunt about it. Among 69,681 people who already had coronary heart disease, carriers and non-carriers had the same rate of heart attack and coronary death, with no difference in stroke, procedures to reopen heart arteries, or death from any cause.

Older pooled analyses found a faint signal, on the order of 17% higher coronary risk per copy across 66,155 cases and 91,307 controls, and an arterial-event estimate that didn't reach significance overall. Set against the venous findings, that's a small effect at best. If arterial risk is what worries you, this isn't the test that answers it.

A Test You Take Once

Your DNA doesn't change, so neither does this result. A clinical DNA test can be done at any time of day, fasting or not, on or off medication, and it does not need repeating. The reason to test again is if the first answer came from a functional screening assay rather than a DNA test, or from a consumer genotyping chip.

What does need revisiting is everything around it. The useful cadence for a carrier isn't retesting the gene, it's reopening the risk conversation at the moments that matter: before starting estrogen, when planning a pregnancy, before elective surgery, after a long stretch of immobility, and after a new cancer diagnosis.

What a Positive Result Should Change

First, pin down what you actually have. A DNA test tells you whether you carry one copy or two, and that distinction matters more than anything else on the report. A positive functional screen, or a consumer chip result, is worth confirming with clinical-grade DNA testing before you build decisions on it.

Then look at the rest of the clotting picture rather than this variant alone, because these defects coexist and compound each other. When testing is indicated, the 2023 American Society of Hematology guidance is to run thrombophilia testing as a panel: prothrombin G20210A, protein C and protein S activity, antithrombin activity, and antiphospholipid antibodies, which are the acquired members of the group. MTHFR variants don't belong in the panel. Factor VIII, IX, and XI levels aren't part of the standard workup either, and a positive MTHFR result mostly generates anxiety.

A hematologist is worth involving if you carry two copies, if you carry this plus another variant, if you've already had a clot, or if you're planning a pregnancy. A genetic counselor is most useful for the family side of the conversation.

And have that conversation. Siblings of someone with two copies are among the groups guidelines specifically name for testing, and first-degree relatives of carriers carry a real share of the risk: pooled across studies, relatives with one copy had about 3.5 times the clot risk of non-carriers, and those with two copies about 18 times.

When the Result Can Mislead

• DNA test versus functional screen: the screening assay measures how sluggishly your blood responds to its natural clotting brake, and blood thinners distort it in both directions, pushing results toward a false positive or masking a real one depending on the drug and on which version of the assay the lab runs. Lupus anticoagulants, pregnancy, oral contraceptives, liver problems, and acute inflammation can interfere too, mostly with the older first-generation version of the test. The DNA test is untouched by every one of these.
• A negative result doesn't clear you: this test looks for one specific change. Other changes in the same gene, and several acquired conditions, also produce resistance to activated protein C, and plenty of clotting disorders have nothing to do with this gene. In people with venous clots, the variant accounts for only a minority of cases, though the share is higher in selected groups with young, recurrent, or family-linked clots.
• Ancestry changes what a result means: the variant is carried by about 5.3% of white Americans, 2.2% of Hispanic Americans, 1.25% of Native Americans, 1.2% of African Americans, and 0.45% of Asian Americans, and it's close to absent in indigenous Australian populations. A negative result rules out less of your inherited risk if the variant was unlikely in your background to begin with.
• Broad genetic reports can blur the question: this test should come back as a clear yes-or-no call for the known F5 Leiden variant, with one copy or two. A variant of uncertain significance in F5 is not the same thing and should not be treated as a positive result.
• Tumor or donor-derived DNA: tumor-only sequencing can report DNA changes that are not inherited. After a donor stem-cell transplant, blood DNA may reflect the donor, not you. In those situations, confirm with a clinical inherited-DNA test using the right sample.
• Consumer reports versus clinical labs: a genotype call from a direct-to-consumer service isn't the same as a clinical-grade test, and an unexpected result there should be confirmed before it drives any decision about hormones, pregnancy, or surgery.