Thrombophilia screening (clotting tendency test) (Thrombophilia testing)
Blood tests that look for an inherited or acquired tendency to form clots. They are only useful in selected situations, because in most people the result does not change treatment.
✓ Medically reviewed by a GMC-registered consultant · last reviewed September 2026 · next review September 2027 · how we review
In short
- Thrombophilia screening looks for an inherited or acquired tendency to form clots, but it is only useful in selected situations.
- For most people who have had a clot, the result does not change treatment or how long blood thinners are needed.
- A normal result does not mean you cannot have a clot, and a positive result can cause unnecessary worry if over-interpreted.
- Testing should be decided by a specialist, with clear counselling, only when the result will genuinely change care for you or a relative.
A plain-English summary. The detail — including risks and recovery — is below.
At a glance
A general guide. Your specialist will give you advice for your situation.
Can occasionally explain why clots have happened, especially when they are unusual or run strongly in a family
The result would not change your treatment or how long you take blood thinners — true for most people who have had a clot.
The specialist confirms whether testing is appropriate and, if so, takes one or more blood samples.
Clear counselling before and after testing about what the result does and does not mean.
The specialist confirms whether testing is appropriate and, if so, takes one or more blood samples.
You can go home straight away. There are no physical after-effects beyond perhaps a small bruise.
Most results take from one to several weeks. Some tests are sent to specialist laboratories.
Antiphospholipid tests usually need a repeat sample after about 12 weeks before a result can be confirmed.
What is thrombophilia screening?
Thrombophilia means a tendency for the blood to clot more easily than usual. It can be inherited (for example factor V Leiden, a prothrombin gene change, or a deficiency of the natural anticoagulants protein C, protein S or antithrombin) or acquired (most importantly antiphospholipid syndrome).
A thrombophilia screen is a set of blood tests looking for these. Importantly, it is only helpful in carefully chosen situations. In most people who have had a clot, the result does not change how they are treated or how long they take blood-thinning medicine, and a 'normal' result does not mean a clot cannot happen again.
The test can sometimes guide decisions for close relatives, for example a young woman thinking about pregnancy or the contraceptive pill in a family with a strong, high-risk clotting condition. But it can also cause worry, affect how people see their own health, and lead to unnecessary treatment if misused.
This guide explains when testing may help, when it is the wrong test, and why careful counselling matters. It is not a treatment.
Types, options & approaches
There may be different ways to do this. The right approach depends on the clinical question and your circumstances.
Options at a glance
These are the main approaches described in this guide. The right option depends on the diagnosis, your goals and what your clinician thinks is safe.
Inherited thrombophilia tests
Look for factor V Leiden and the prothrombin gene change (common, usually low-risk) and for deficiencies of protein C, protein S or antithrombin (rarer, can be higher-risk).
Antiphospholipid (acquired) tests
Look for antiphospholipid syndrome, which can cause clots and pregnancy problems. These tests usually need repeating after about 12 weeks to confirm, and timing matters.
Selective testing of relatives
Sometimes offered to close relatives of someone with a strong, high-risk inherited condition, but only when the result would genuinely change that person's choices.
Genetic versus clotting-protein tests
Some are genetic (DNA) tests; others measure the level or activity of a clotting protein. Levels can be affected by a recent clot, pregnancy and blood-thinning medicines, so...
Preparing for your test
- Ask first why the test is being suggested and whether the result will actually change your care — in most cases it does not.
- Tell the team if you are taking blood thinners (warfarin, apixaban, rivaroxaban and others), as these can make some results unreliable.
- Mention if you have recently had a clot, are pregnant or have recently given birth, as timing affects the results.
- Bring details of any personal and family history of clots, including how old people were and whether there was a trigger.
- Ask whether testing should wait until after treatment for a current clot has finished.
- Ask what counselling and support are available before and after the test, especially genetic results.
- Consider who might see the result and any implications for relatives before you go ahead.
What happens
A specialist first decides whether testing is appropriate, because for most people it is not needed. If it is, one or more blood samples are taken from your arm. This usually takes only a few minutes.
Some results, especially antiphospholipid tests, need a second sample after about 12 weeks to confirm. Levels of natural anticoagulants can be temporarily altered by a recent clot, pregnancy or blood thinners, so testing is often delayed until the timing is right and you are off treatment, if that is possible and safe.
When the results are back, the specialist explains what they mean for you and, where relevant, for your relatives. Good practice includes clear counselling so you understand that most results do not change treatment and that a normal result does not remove all risk.
Is this test right for me?
A good consultation should explore whether it's the right choice for you now — including reasons to wait or consider something else.
May not be suitable if…
- The result would not change your treatment or how long you take blood thinners — true for most people who have had a clot.
- You are looking to 'check your clot risk' while well, with no clear personal or family reason.
- You are on blood thinners or have recently had a clot or given birth, when many results are unreliable.
- It is being used to assess arterial problems (such as most strokes or heart attacks), where these tests are generally irrelevant, except for lupus anticoagulant.
- Routine testing of children, where it is rarely appropriate.
Delay or rearrange if…
- You are still on blood-thinning treatment that would make results unreliable.
- You have had a clot or given birth recently, so levels are temporarily altered.
- You are pregnant, when interpretation and timing need specialist input.
- Proper counselling and a clear plan for using the result are not yet in place.
- Treatment for a current clot has not yet finished and testing can safely wait.
Alternatives to discuss
- No testing, which is the right choice for most people after a clot.
- Basing treatment decisions on whether the clot had a trigger and your overall risk, rather than on a test.
- Selective, targeted testing of one relevant condition instead of a full screen.
- Specialist counselling alone to discuss family risk without testing.
- Reassessing risk at key times (such as before pregnancy or surgery) rather than testing now.
Before you decide
Use this as a shared-decision checklist. The aim is not just “can this be done?”, but whether it is right for you, now, with the risks and alternatives clearly understood.
What matters most to me?
Think about symptoms, daily life, work, caring responsibilities, sport, fertility, travel, appearance and anxiety — the right choice depends on your priorities, not just the medical facts.
What are all my options?
Ask about waiting, monitoring, medicines, rehabilitation, a smaller or larger procedure, a different test, NHS referral, or a second opinion where that would help.
What would make me pause?
Active infection, pregnancy, unstable medical problems, smoking, medicines that increase bleeding, poor support at home, or feeling pressured are all reasons to slow down and get tailored advice.
What happens if I do nothing today?
For some problems, waiting is safe; for others, delay can make treatment harder. A good consultation should explain the trade-off in plain English.
Benefits
- Can occasionally explain why clots have happened, especially when they are unusual or run strongly in a family
- May guide choices for close relatives in families with a strong, high-risk condition
- Can inform decisions around pregnancy or the contraceptive pill in selected people
- Can identify antiphospholipid syndrome, which sometimes changes treatment
- Can reassure when there is a clear, agreed reason to test and the result is acted on sensibly
Risks & complications
- Discomfort or a small bruise from the blood test
- Worry or anxiety created by being labelled with a clotting tendency
- A result that does not change your treatment at all
- False reassurance from a normal result, which does not remove future clot risk
- Unnecessary or longer blood-thinning treatment if the result is over-interpreted
- Misleading results caused by testing at the wrong time or while on blood thinners
- Implications and anxiety for relatives who learn of an inherited result
- Effects on how you feel about your health, pregnancy plans or contraception
- Possible effects on some insurance or financial decisions if results are disclosed (ask about this)
- Important harm if a result wrongly drives a major decision without specialist input
The main harm from thrombophilia testing is not the blood test — it is testing when there is no good reason, and then acting on the result in ways that do not help. Results rarely change treatment, can cause lasting worry, and can affect relatives. Ask exactly how a positive or negative result would change your care before agreeing to the test.
Published figures to discuss
Thrombophilia tests do not have a meaningful 'complication rate' beyond the minor bruising of a blood test. The more important point is how often the result is actually useful. UK guidance is clear that these tests are poor at predicting whether a clot will come back and, in almost all patients, do not change current or future treatment. Because the value depends entirely on the individual situation, we describe it in words rather than giving fixed percentages.
| Figure | Reported range | How to interpret it | Source / confidence |
|---|---|---|---|
| Test does not explain most clots | Common | Age, surgery, immobility, pregnancy, cancer and hormones often matter more than inherited thrombophilia tests. | Guide sourcesClinical context |
| False or misleading result during acute clot/anticoagulation | Recognised | Some assays are affected by warfarin, DOACs, heparin, pregnancy or the acute event itself. | Guide sourcesClinical context |
| Result changes management | Often limited | Testing is most useful when it will alter anticoagulation duration, family advice or pregnancy/hormone decisions. | Guide sourcesClinical context |
| Family anxiety or insurance implications | Consent issue | Testing can affect relatives and should be discussed before ordering. | Guide sourcesClinical context |
These are literature figures, not a personalised prediction. Your own risks and likely benefits depend on your circumstances, your health, and how your care is carried out and followed up.
What happens afterwards
There is no physical recovery. What matters afterwards is making sense of the result with proper counselling, and not making big decisions based on a test that, for most people, does not change treatment.
- A small bruise where blood was taken
- Waiting weeks for some results, with a repeat needed for certain tests
- Being told the result does not change your treatment, which is common and expected
- Some mixed feelings about being given, or not given, a label
Aftercare
- Make sure you understand whether the result changes anything for you — often it does not.
- If you are on blood thinners, keep taking them as directed and do not stop based on a test result without specialist advice.
- Discuss with your specialist before sharing results with relatives, and ask about counselling for them.
- Keep a copy of your result and the explanation for future reference.
- Ask how the result affects decisions such as pregnancy, the pill or HRT, if relevant.
- Remember that a normal result does not remove the need to act quickly if you get clot symptoms.
- Ask about any implications for insurance or financial decisions before disclosing results.
- A clear written reason for testing and how the result will be used
- Your medicines list, including any blood thinners
- Details of personal and family clotting history
- A note of whether and when a repeat sample is needed
- Information about counselling and support, especially for genetic results
- A way to discuss results with relatives sensibly, if relevant
⚠ Get urgent help if…
- A leg that is increasingly painful, swollen, hot or changing colour (possible DVT)
- Sudden breathlessness or chest pain that is worse on breathing in (possible lung clot)
- Coughing up blood, fainting or a fast heartbeat
- These need urgent assessment regardless of any thrombophilia result
- New or worsening symptoms in pregnancy, which should be reported promptly
- Severe anxiety or distress after a result — ask for support and counselling
Who to contact: your clinician, clinic or test provider first (keep their number to hand). For urgent advice when you can't reach them, call NHS 111. In an emergency, call 999.
General guidance — it doesn't replace the specific advice your specialist gives you.
Results & realistic expectations
A useful result is one that has been agreed in advance to change something specific — for example informing a relative's choices about pregnancy or contraception, or identifying antiphospholipid syndrome that alters treatment. For most people who have had a clot, the result does not change how they are treated or how long they take blood thinners.
A negative (normal) result does not mean you cannot have a clot. It only means the specific things tested for were not found. This is why current UK guidance stresses testing only when the result will genuinely affect care, and why counselling is important before and after.
A genetic result does not change over your lifetime, but its meaning depends on your overall situation, which can change. Clotting-protein levels and antiphospholipid results can vary with time, treatment and pregnancy, which is why some tests need careful timing or repeating. The result should always be interpreted alongside your personal and family history.
Related tests, treatments or support
Thrombophilia testing is sometimes considered after a clot has been confirmed and assessed (see the blood clotting assessment guide), but only in selected cases. It is not part of a routine clot work-up for most people. In pregnancy or recurrent miscarriage, decisions about testing are made with the relevant specialists.
Follow-up & long-term care
If you are tested, you should have a clear plan for how and when results are explained, including any repeat samples. Results should be discussed with proper counselling, and you should be told plainly whether anything in your care actually changes.
Repeat, follow-on and what comes next
- Some tests, especially antiphospholipid, need repeating after about 12 weeks to confirm a result.
- Results taken at the wrong time or on blood thinners may need to be repeated when timing is better.
- A result rarely changes management, so 'revision' is usually about re-checking timing rather than changing treatment.
Ask what happens if the result is unclear or needs repeating, and what is included if further tests or follow-up are needed.
What good aftercare looks like
- Clear counselling before and after testing about what the result does and does not mean.
- A documented plan stating whether anything in your care changes.
- Sensible advice on discussing results with relatives, with onward referral if needed.
- A reminder that clot symptoms still need urgent attention whatever the result.
- A named contact for questions and support after the result.
What affects the cost
Costs vary a great deal between people and providers, and we don't publish prices. What matters is understanding what drives the cost and making sure your quote is complete. The main things that affect it:
- The specialist consultation to decide whether testing is appropriate
- Which tests are done — a full screen costs more than a single targeted test
- Specialist laboratory analysis, including genetic tests
- Repeat samples needed for some tests (for example antiphospholipid)
- Counselling, particularly for genetic results
- A follow-up appointment to explain results and any implications for relatives
- The specialist's fee for the consultation and decision to test
- Exactly which tests are included and which are extra
- Laboratory and genetic test fees
- The cost of any repeat samples needed to confirm a result
- Counselling and follow-up to explain results
- What happens if the result does not change your care
- The cancellation policy
On the NHS? Thrombophilia testing is available on the NHS in selected cases decided by a specialist; it is rarely needed routinely, and private testing should not be done without a clear reason or proper counselling.
You're entitled to your total cost in writing — including reports, follow-up and what happens if the result is inconclusive — before you decide.
Consent traps and marketing red flags
These are not small details. They are often where patients lose choice, time to reflect, or realistic expectations.
Consent traps
- Not explaining that, for most people, the result will not change treatment.
- Implying a normal result means there is no future clot risk.
- Testing without counselling about the effect on you and your relatives.
- Testing at the wrong time or on blood thinners, giving misleading results.
- Not mentioning possible implications for insurance or financial decisions before disclosure.
Marketing red flags
- Promoting a thrombophilia 'screen' to healthy people as a general wellness check.
- Suggesting everyone who has had a clot should be tested.
- Implying the test can reliably predict whether you will clot again.
- Offering testing without specialist assessment or counselling.
- Using a normal result to give blanket reassurance that you are 'safe'.
Choosing a specialist safely
- Check the specialist is on the GMC Specialist Register for this area.
- Make sure they work at a CQC-registered service, and look for membership of the relevant Royal College or professional body.
- You're entitled to time to consider and to have your questions answered before you agree — the specialist who looks after you should explain it, not a salesperson.
- Be wary of pressure: time-limited offers or deposits taken before you've had time to think are red flags, not bargains.
- You're entitled to your total cost in writing — including any follow-up — before you decide.
Questions to ask your medical professional
Take this to your consultation. A good specialist will welcome every one of these.
- Why are you suggesting this test, and exactly how would a positive or negative result change my care?
- Is testing actually recommended in my situation, or is it unlikely to help?
- Should we wait until I am off blood thinners or past a recent clot or pregnancy?
- What counselling and support is available, for me and for any relatives?
- Could a result affect my relatives, my pregnancy plans, or insurance decisions?
- If the result is normal, does that change anything about my risk or my plan?
- Are you on the GMC Specialist Register for this area, and which Royal College or professional body are you a member of?
- Will you be the specialist who carries out my test, and who looks after me afterwards?
- What are the risks for someone like me, and how often do your own patients have a problem or need it repeated or redone?
- What does a realistic result look like — and what can this test not achieve?
- What are my options, including waiting, doing nothing for now, or choosing a different approach?
- Can I have written information, results and aftercare instructions in a format I can use, including any accessibility or communication support I need?
- What is the total cost in writing, including any follow-ups, and how much time do I have to decide?
Frequently asked questions
Should everyone who has had a clot be tested for thrombophilia?
If I have factor V Leiden, do I need lifelong blood thinners?
Does a normal result mean I will not get a clot?
Should my children or relatives be tested?
Can I be tested while on blood thinners?
Could a result affect my insurance?
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How we made this page
Medically reviewed by a GMC-registered consultant. Written in plain English, checked against NHS, NICE, GMC and relevant Royal College / specialist-society guidance, and kept under review. No clinic paid to appear on this page, and we publish no pricing. This is general information to help you prepare — it is not a substitute for advice from your own clinician. How we review our guides →
Source hierarchy: UK regulator and NHS/NICE guidance first, then relevant Royal College or specialist-society guidance, then peer-reviewed evidence for procedure-specific figures where available.
Sources & standards: NICE NG158 — Venous thromboembolic diseases: diagnosis, management and thrombophilia testing NICE NG158 — Recommendations (thrombophilia testing) British Society for Haematology — Thrombophilia testing guideline (2022) British Society for Haematology — Guidelines for thrombophilia testing NHS — Von Willebrand disease (haematology referral context)
Reviews reflect patients' experience of care, not clinical outcomes. For procedure volumes and outcome data see PHIN.
Last medically reviewed 2026-09-21. Spotted something wrong or out of date? Report an error in this guide.
Related guides: Blood clot (thrombosis) assessment · DVT treatment · Anticoagulation (blood-thinning treatment) · Bleeding disorder assessment · Full blood count review