A Positive Test, No Clot, and a Sister's Recent PE: What the Result Actually Obligates
Cascade testing after her sister's unprovoked pulmonary embolism found the same mutation in her — and now she wants to know what, if anything, a healthy person is actually supposed to do with that information.
N.R., a 27-year-old marketing associate, has never had a blood clot, never been hospitalized, and would not have thought to get tested for anything if her older sister hadn't been admitted three weeks ago with an unprovoked pulmonary embolism that prompted a thrombophilia workup, which came back positive for heterozygous Factor V Leiden. Cascade testing offered to first-degree relatives found the same mutation in N.R., and she came to today's appointment less alarmed than confused, asking directly what a positive result is actually supposed to change for someone who has never had a symptom and feels, by her own account, completely fine.
Heterozygous Factor V Leiden is the most common inherited thrombophilia, present in roughly five percent of the general population of European descent, and, as the 2023 ASH thrombophilia testing guidelines set out, a real but modest risk factor — activated protein C resistance from the mutation raises relative VTE risk several-fold, but the absolute annual risk for an asymptomatic heterozygous carrier remains low, well under one percent per year in most estimates, nowhere near the risk profile that would justify continuous anticoagulation as primary prevention in someone who has never clotted. She is not currently pregnant, is on a combined oral contraceptive she started two years ago without any problem so far, and has no upcoming surgery planned — the three situations where her carrier status would concretely change standard medical advice are all, for the moment, hypothetical rather than active.
Her sister, still recovering from the PE and the anticoagulation that followed, has been the one pushing hardest for N.R. to 'do something' — an understandable reaction from someone who has just been frightened by her own diagnosis, but one the hematologist is careful not to let substitute for what N.R.'s own risk profile actually supports.
N.R. herself trains for half-marathons most weekends and has an international trip already booked for next spring — both facts that came up naturally in conversation and that the hematologist folded directly into the written situational-prophylaxis plan, rather than leaving her to wonder later whether either one counted as a moment her carrier status was supposed to change something.
In clinic, three weeks after her sister's hospitalization
The honest answer for most of what you're asking is: nothing changes today. Heterozygous Factor V Leiden raises relative risk several-fold, but your absolute annual risk as an asymptomatic carrier stays low, well under one percent most years. No guideline recommends continuous anticoagulation for someone in your situation who has never clotted — the right approach is being prepared for specific future situations, like surgery or pregnancy, not treating today any differently.
I'd flag one thing that isn't hypothetical, though — you're on a combined oral contraceptive right now. Estrogen and Factor V Leiden combine synergistically, not just additively, and that's the one exposure in your current life this result should genuinely prompt us to revisit today rather than file away for later.
That's a real, separate point from continuous anticoagulation — it's about which form of contraception makes sense for you now, not about starting a blood thinner.
I want to name something underneath the question you actually asked — your sister scared you, understandably, and it's natural to want to do something concrete in response. I think the honest, complete answer includes both of what's been said: the contraception conversation is real and worth having today, and for everything else, reassurance isn't us brushing you off, it's the medically correct response to your actual risk level. You're not being told to ignore this — you're being told what it does and doesn't change.
Agreed: no continuous anticoagulation started. She was given a written situational-prophylaxis plan to carry to any future surgeon or obstetrician, and left the visit deciding to switch to a progestin-only contraceptive at her own pace rather than immediately, after discussing it directly with the hematologist.