Blood clot (thrombosis) assessment
A set of checks — using your symptoms, a risk score, a blood test and usually a scan — to find out whether you have a blood clot in a deep vein (DVT) or lung (pulmonary embolism), or to work out why one happened.
✓ Medically reviewed by a GMC-registered consultant · last reviewed September 2026 · next review September 2027 · how we review
In short
- It is a structured way to find out whether you have a clot in a deep vein (DVT) or lung (PE), using your symptoms, a risk score, a blood test and usually a scan.
- A normal D-dimer blood test with a low or moderate risk score can rule a clot out, but a raised result does not confirm one and a scan is often needed.
- A suspected clot is urgent — use NHS emergency routes straight away rather than waiting for a private appointment.
- If a clot is found, the team also works out why it happened, because that affects how long treatment is needed.
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 confirm or rule out a blood clot quickly so treatment is not delayed
You have signs of a serious, life-threatening clot (severe breathlessness, chest pain, collapse) — this needs emergency care, not a planned assessment.
You answer questions, are examined, and have a blood test. You may wait for a scan, sometimes with a precautionary dose of blood thinner.
A clear, documented result and plan, with an explanation you understand.
You answer questions, are examined, and have a blood test. You may wait for a scan, sometimes with a precautionary...
The risk score and blood test are usually available the same day. An ultrasound result is often available straight...
If a scan could not be done immediately, it is usually arranged within 24 hours, with treatment cover in the...
Treatment with blood-thinning medicine usually starts straight away, and the team explains how long it is likely...
What is a blood clot (thrombosis) assessment?
A blood clot in a deep vein (deep vein thrombosis, or DVT) is most common in the leg. If part of it breaks off and travels to the lungs it causes a pulmonary embolism (PE), which can be serious. Together these are called venous thromboembolism (VTE).
A blood clotting assessment is the process clinicians use to decide whether you have a clot. It usually starts with your symptoms and a scoring system (such as the Wells score) that estimates how likely a clot is. Depending on the score, you may have a blood test called a D-dimer and, in many cases, a scan such as an ultrasound of the leg or a CT scan of the lungs.
The assessment tells you whether a clot is likely and whether a scan has found one. After a clot is confirmed, doctors also think about why it happened — for example a recent operation, long journey, the contraceptive pill or HRT, pregnancy, cancer or an inherited tendency.
It is not a treatment. If a clot is found, treatment is usually with blood-thinning medicine, which is covered in a separate guide.
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.
Clinical risk score (such as the Wells score)
A short checklist of your symptoms and risk factors that estimates how likely a clot is. It guides whether you need a blood test, a scan, or both.
D-dimer blood test
Measures a substance released when a clot breaks down. A normal result, combined with a low or moderate risk score, can help rule out a clot. It is often raised for reasons...
Ultrasound scan of the leg
The usual scan to look for a DVT. It is usually not painful and uses sound waves to check whether blood is flowing normally through the deep veins.
CT pulmonary angiogram (CTPA)
A CT scan of the lungs with an injection of contrast dye, used to look for a pulmonary embolism. A ventilation–perfusion (V/Q) scan is sometimes used instead, for example in...
Preparing for your test
- Do not delay seeking help — a suspected clot is urgent. Call 999 or go to A&E if you are very breathless or have chest pain. For urgent advice that is not life-threatening, use NHS 111 in England, Scotland or Wales; in Northern Ireland contact your GP out-of-hours service or your HSC Trust's Phone First service. Do not wait for a private appointment.
- Write down when your symptoms started and how they have changed.
- Note any recent triggers: surgery, a long journey, illness, immobility, pregnancy or recent birth.
- List your medicines, including the contraceptive pill, HRT and any blood thinners.
- Tell the team if you might be pregnant, as this affects which scan is used.
- Mention any allergy to contrast dye and any kidney problems before a CT scan.
- Bring details of any previous clots and any family history of clots.
What happens
A clinician asks about your symptoms and risk factors and examines you, then works out a risk score. Based on this, they decide what to do next.
If a clot is unlikely on the score, you may have a D-dimer blood test. A normal result can be enough to rule a clot out without a scan. If the score suggests a clot is likely, or the blood test is raised, you will usually go on to have a scan.
For a suspected leg clot this is normally an ultrasound; for a suspected lung clot it is usually a CT scan of the lungs (sometimes a V/Q scan). While waiting for the scan, you may be given a starting dose of blood-thinning medicine to be safe.
If a clot is confirmed, the team explains treatment and starts to look at why it happened. If nothing is found, they explain what else might be causing your symptoms.
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…
- You have signs of a serious, life-threatening clot (severe breathlessness, chest pain, collapse) — this needs emergency care, not a planned assessment.
- Your symptoms point clearly to a different problem that needs a different test or specialist.
- You are being asked to have thrombophilia (clotting tendency) tests routinely when they will not change your care.
- A scan is being requested when the risk score and blood test have already safely ruled a clot out.
Delay or rearrange if…
- Urgent emergency care is needed first for breathlessness, chest pain or collapse.
- You might be pregnant and the safest scan choice needs to be confirmed.
- A contrast allergy or kidney problem needs checking before a CT scan.
- Key information, such as recent results or your medicines list, is missing.
- You are too unwell for a non-urgent private pathway and should use emergency NHS care.
Alternatives to discuss
- Emergency NHS assessment, which is the right route for a suspected clot.
- Risk score and D-dimer alone when these safely rule a clot out.
- An ultrasound rather than a CT scan for a suspected leg clot.
- A V/Q scan instead of CT in pregnancy or contrast allergy.
- Watchful waiting with clear safety advice when a clot is very unlikely.
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 confirm or rule out a blood clot quickly so treatment is not delayed
- Uses a structured approach so the right test is done at the right time
- Can avoid an unnecessary scan when a clot is unlikely and the blood test is normal
- Helps find why a clot happened, which guides how long treatment is needed
- Gives you a clear plan and explanation of your symptoms
Risks & complications
- Discomfort or a small bruise from the blood test
- Anxiety and uncertainty while waiting for results
- A raised D-dimer caused by something other than a clot, leading to a scan you may not have needed
- Time spent in a clinic or assessment unit
- A normal result that does not fully settle symptoms, so further assessment is needed
- Radiation exposure from a CT scan (low, but worth knowing)
- A reaction to the contrast dye used in a CT scan
- A clot missed on early scanning that shows up on a repeat scan
- A serious allergic reaction to contrast dye
- A significant delay or wrong decision if the assessment is rushed or incomplete
- Bleeding from blood-thinning medicine started while awaiting results
The biggest risks are getting the decision wrong: missing a clot, or over-treating a false alarm. The D-dimer test is good at ruling clots out when used with a risk score, but a high result is common and does not prove a clot. Ask how confident the team is, whether a scan is needed, and what to do if your symptoms change or get worse.
Published figures to discuss
This is an assessment, not a treatment, so the main 'risks' are about getting the answer right. The D-dimer test is most useful for ruling a clot out when combined with a clinical risk score; on its own it is often raised by other things. Scans are very accurate but not perfect, and an early scan can occasionally miss a small clot. Exact accuracy figures vary by the test used, the population and how soon after symptoms the test is done, so we describe them in words rather than as fixed percentages.
| Figure | Reported range | How to interpret it | Source / confidence |
|---|---|---|---|
| Pulmonary embolism missed | Potentially life-threatening | Chest pain, breathlessness, collapse or coughing blood with possible DVT symptoms needs urgent assessment. | Guide sourcesClinical context |
| D-dimer false positive | Common in many illnesses | Age, inflammation, cancer, pregnancy, infection and recent surgery can raise D-dimer without a clot. | NHS — DVT (deep vein thrombosis)nhs.ukSource-linked context |
| D-dimer false reassurance if used incorrectly | Avoidable | D-dimer is interpreted with clinical probability; high-risk symptoms may need imaging even if tests are awkward. | NHS — DVT (deep vein thrombosis)nhs.ukSource-linked context |
| Bleeding from anticoagulation | Medicine- and patient-dependent | Treatment decisions balance clot recurrence risk against bleeding, kidney function, interactions and patient preference. | NHS — DVT (deep vein thrombosis)nhs.ukSource-linked 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 from the assessment itself. What matters afterwards is getting a clear result, understanding what it means, and knowing what to do next — including starting treatment quickly if a clot is found.
- A small bruise or tenderness where blood was taken
- Feeling anxious or unsettled while waiting for results
- Being asked to return for a repeat scan in a day or two in some cases
- No physical after-effects from the assessment itself
Aftercare
- Make sure you understand your result and the plan before you leave.
- If a clot is confirmed and you are started on blood thinners, take them exactly as directed and never stop suddenly without advice.
- If no clot is found but symptoms continue or worsen, seek help again — clots can sometimes show on a later scan.
- Know the warning signs of a clot reaching the lungs and act urgently if they occur.
- Keep any follow-up or repeat-scan appointment.
- Ask whether anything (such as the pill or HRT) should change while things are sorted out.
- Save the contact number for the assessment unit or clinic.
- A clear note of when symptoms started and any triggers
- Your up-to-date medicines list, including the pill, HRT and blood thinners
- Details of any previous clots and family history
- Information about any contrast allergy or kidney problems
- A way to get to A&E quickly if you become very breathless or have chest pain
- The assessment unit or clinic contact number saved
⚠ Get urgent help if…
- Sudden breathlessness or shortness of breath
- Chest pain that is worse when you breathe in
- Coughing up blood
- A fast heartbeat, feeling faint or collapsing
- A leg that is increasingly painful, swollen, hot or changing colour
- If you have started blood thinners: heavy or unusual bleeding that will not stop
- Feeling rapidly more unwell while waiting for a scan or result
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 good outcome from the assessment is a clear, confident answer: either a clot is found and treatment starts promptly, or a clot is ruled out and your symptoms are explained. When a risk score is low or moderate and the D-dimer is normal, a clot can often be safely ruled out without a scan. When a scan is done, it usually gives a definite answer.
No single test is perfect. A normal D-dimer does not rule out every clot in high-risk people, and an early scan can occasionally miss a small clot that shows on a repeat scan. That is why the assessment combines several pieces of information and why you are told what to do if symptoms change.
The result reflects your situation at the time. A clot ruled out today does not protect you in future, so the risk should be reassessed if you develop new symptoms or face a new risk (such as surgery, immobility or pregnancy). If a clot is confirmed, decisions about how long to treat are reviewed over the following weeks and months.
Related tests, treatments or support
The parts of the assessment — risk score, D-dimer and scan — are designed to be used together, not alone. After a confirmed clot, your clinician may consider further tests for an underlying cause, including, in selected cases, thrombophilia testing or checks for an associated condition such as cancer. These are decided individually.
Follow-up & long-term care
If a clot is found, you will normally be reviewed to plan and adjust treatment and to discuss how long it should continue. If no clot is found, you should be told clearly what to do if symptoms persist or return, and when a repeat scan or further assessment is needed.
Repeat, follow-on and what comes next
- If an early scan is normal but suspicion remains, a repeat scan in a day or two is sometimes advised.
- A raised D-dimer often leads to a scan that turns out normal — this is expected, not a failure of the test.
- After a confirmed clot, treatment length is reviewed and may be adjusted as more is learned about the cause.
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
- A clear, documented result and plan, with an explanation you understand.
- Written safety advice on the warning signs of a lung clot and what to do.
- A named contact route and a plan for any repeat scan or review.
- If treatment is started, clear instructions on the medicine and not stopping it suddenly.
- Onward referral or further tests arranged where a cause needs investigating.
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 clinician's assessment and time
- The D-dimer and any other blood tests
- The type of scan needed (ultrasound, CT or V/Q) and the reporting specialist
- Whether contrast dye is used
- Any blood-thinning medicine started while awaiting results
- Follow-up appointments and further tests to look for a cause
- Whether a repeat scan is needed
- The clinician's fee for the assessment
- Blood tests, including the D-dimer
- Each scan and its reporting fee, including contrast if used
- Any medicines started during the assessment
- Follow-up appointments and any further tests
- What happens, and what it costs, if a repeat scan or further assessment is needed
- The cancellation policy
On the NHS? Suspected clots are assessed urgently on the NHS, usually with referral to hospital within 24 hours; private routes are mainly for non-urgent review or a second opinion and should not delay urgent care.
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 a raised D-dimer is common and does not prove a clot.
- Doing thrombophilia tests routinely without explaining they often do not change care.
- Not giving clear safety advice on what to do if symptoms worsen after a normal result.
- Starting blood thinners without explaining the bleeding risk and the plan.
- Not telling you which scan is used and why, including in pregnancy.
Marketing red flags
- Offering a private 'clot screen' as a substitute for urgent care when you have warning symptoms.
- Promoting routine thrombophilia testing to healthy people as a way to 'check your clot risk'.
- Suggesting a single blood test can confirm or exclude a clot on its own.
- Implying a normal result means you can never have a clot in future.
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.
- How likely do you think a clot is, and what is my risk score?
- Do I need a scan, and which one — and what does the result change?
- What happens if the result is normal, abnormal or unclear?
- Should I start blood-thinning medicine while I wait for results?
- If a clot is found, how long am I likely to need treatment, and why?
- What should I do if my symptoms get worse or come back?
- 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 I wait for a private appointment if I think I have a clot?
What is a D-dimer test?
Will I always need a scan?
Is the CT scan safe?
Why do they want to know why the clot happened?
Do I need a thrombophilia (clotting tendency) test?
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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: NHS — DVT (deep vein thrombosis) NICE NG158 — Venous thromboembolic diseases: diagnosis, management and thrombophilia testing NICE NG158 — Information for the public NICE NG158 — Recommendations Thrombosis UK — Patient information resources nidirect — Urgent and emergency care services nidirect — GP out-of-hours service
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: DVT treatment · Anticoagulation (blood-thinning treatment) · Thrombophilia screening (clotting tendency test) · Bleeding disorder assessment · Full blood count review