Why this might matter to you
Factor V Leiden usually arrives with a frightening number attached. It's the variant described as raising the risk of a blood clot roughly 5 to 10 times over, and a multiplier like that reads as a sentence rather than a statistic PMID 21116184. Here is the part the number leaves out: in the published data, the large majority of people who carry one copy never have a clot at all PMID 21116184. Both of those things are true at once.
The biology underneath is concrete. Factor V Leiden is a common inherited change in F5, the gene for a blood-clotting protein called Factor V. Your blood clots when you're injured and switches that clotting off when the job is done, and Factor V Leiden slows the off switch - the brake that would normally shut the clotting signal down. That much is established, well-documented biology. What it doesn't do is hand you a diagnosis.
What it does and doesn't change
A useful way to read the risk is to separate the multiplier from the baseline. In one group there are 100 people of European descent carrying one copy of Factor V Leiden; across a lifetime, roughly 10 of them have a venous blood clot, and roughly 90 do not PMID 21116184. The "5 to 10 times" figure is real, but it's a multiplier on a baseline that starts low, so a large relative jump still leaves most carriers clot-free. The variant loads the dice. It doesn't throw them.
Two more facts keep the picture honest. The clotting risk here is venous - clots that form in veins, such as a deep vein thrombosis in the leg or a pulmonary embolism in the lung. The prospective study that first established the association in healthy people found no measurable rise in heart attack or stroke among carriers PMID 7877648. And the absolute risk is heavily situational. Published literature documents that surgery, prolonged immobility, pregnancy, and estrogen-containing contraceptives each interact with this genotype to raise the short-term odds well above the carrier's resting baseline PMID 21116184. It's a finding clinicians weigh in those specific contexts, not a fixed property that travels with you unchanged.
What the genotype is
Factor V Leiden is read at a single position, rs6025, on chromosome 1. The change swaps one amino acid in the Factor V protein at position 506, arginine for glutamine, written R506Q, and that single swap is what alters how the protein behaves. The reference genotype (C,C) carries no Leiden change. One copy (C,T) is the heterozygous form, the most common inherited clotting variant in people of European descent. Two copies (T,T) is the homozygous form, and it's rare. In Varia's catalog the variant sits in the Cardiovascular and Lipid module, under the Heart and Lipids area.
How common is it? In gnomAD reference data the Leiden allele appears at roughly 2 percent worldwide, and the dataset records 413 homozygous individuals, so the two-copy genotype is uncommon but real and catalogued. The frequency is not even across populations. It's most common in people of European, Middle Eastern, and Ashkenazi Jewish descent, and close to absent in people of East Asian descent, where gnomAD records it in fewer than 1 in 1,000. The catalog puts the one-copy carrier rate in European-descent populations at roughly 3 to 8 percent, and the two-copy rate at roughly 0.06 percent.
What the literature reports
The risk numbers come from two cited papers, and they don't say quite the same thing. Ridker and colleagues (1995), writing in the New England Journal of Medicine, followed a large group of initially healthy men and found that one-copy carriers had about a 3-fold higher rate of venous thrombosis, with no measurable rise in heart attack or stroke PMID 7877648. Kujovich (2011), reviewing the accumulated clinical literature in Genetics in Medicine, characterized the one-copy risk across studies as roughly 5 to 10 times baseline, with an absolute lifetime clot risk near 10 percent, and the rare two-copy genotype at roughly 50 to 80 times baseline PMID 21116184.
Why the spread between "3-fold" and "5 to 10 times"? A single prospective cohort and a review of many studies are answering slightly different questions, in different populations, with different definitions of the outcome. The direction is consistent and the effect is real; the precise multiplier is not a single settled figure. For the two-copy genotype the uncertainty is wider still, because it's rare enough that most estimates rest on small numbers and carry correspondingly wide confidence intervals PMID 21116184.
The mechanism is simpler than the risk range suggests. Factor V, in its activated form, amplifies the signal that builds a clot. The body's brake on that signal is a molecule called activated protein C, which snips activated Factor V at position 506 to switch it off. The Leiden change sits at that exact spot, so the snip works poorly and the activated protein lingers, sustaining the clotting signal longer than it should PMID 21116184. This is why the trait is often called "activated protein C resistance" - the clotting factor resists being switched off.
What Varia does with this variant
Varia reads your raw DNA file on your own device and, where your file includes rs6025, resolves your genotype locally. The result appears under Heart and Lipids, in the Cardiovascular and Lipid module, with the matching catalog summary and the linked, peer-reviewed citations above. We describe what the published evidence reports, including where the estimates disagree. We don't tell you what to take or do, and we won't carry a claim the catalog can't cite. Our intention is to hand you the facts as they stand, without sugarcoating and without fear-mongering.
Questions and answers
Does carrying Factor V Leiden mean I'll have a blood clot?
No. It raises the relative risk, but on a baseline low enough that most one-copy carriers never have a clot. The published lifetime figure is around 10 percent for people with one copy, and the risk climbs mainly during specific situations like surgery, immobility, or pregnancy PMID 21116184. It describes a tendency across groups, not a fate for any individual.
Is one copy the same as two?
No. The catalog describes one copy (C,T) as roughly 5 to 10 times the baseline venous clot risk, and two copies (T,T) as roughly 50 to 80 times, a much steeper and much rarer configuration present in around 0.06 percent of people of European descent PMID 21116184.
Does it raise my risk of heart attack or stroke?
The association here is with venous clots, like deep vein thrombosis and pulmonary embolism. The prospective study that established Factor V Leiden as a thrombosis risk factor found no significant increase in heart attack or stroke among carriers PMID 7877648.
The short version
What is established: Factor V Leiden (rs6025) is a common inherited variant that slows the body's brake on clotting, raising the relative risk of venous blood clots, roughly 5 to 10 times for one copy and 50 to 80 times for the rare two copies, while the absolute lifetime risk for a one-copy carrier sits near 10 percent PMID 7877648 PMID 21116184.
What is not established: that carrying the variant determines any individual's outcome. The risk is a group-level tendency, heavily modified by situation, and the precise multiplier varies between a single prospective cohort and a broader review PMID 7877648 PMID 21116184.
To check your own genotype and read the cited, peer-reviewed evidence behind it, use the Varia Genome Scanner. It reads your raw DNA file in your browser and checks it for rs6025 along with the rest of the catalog. Array files like 23andMe and AncestryDNA cover most catalog variants but not all, so the scan tells you whether yours is included, and your file never leaves your device.