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Clot risk in polycythaemia vera and ET, and how it is scored | CION Cancer Clinics
In polycythaemia vera and essential thrombocythaemia, clot risk is sorted using your age, any past clot and, in ET, your JAK2 gene result. Being over 60 or having had a clot places you in the higher-risk group, which usually means adding a medicine to lower the counts. The score guides treatment. It does not predict whether you will have a clot, or how long you will live. At CION Cancer Clinics, every leukaemia, MDS and MPN case is reviewed by our haematologist and discussed at a tumour board before a plan is agreed.
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Medically reviewed by Dr. Basudev PokhrelConsultant Haematologist · last reviewed September 2026, next review due September 2027
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On this page
- How is thrombosis risk scored in an MPN?
- Which risk groups are used for polycythaemia vera and ET?
- How does your haematologist work out your group?
- What does your group change about treatment?
- What do people misunderstand about clot risk scores?
- What can a risk score not tell you?
- Common questions about MPN clot risk
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If a clot may be happening now
Call 108 or go to the nearest emergency department straight away if there is a drooping face, a weak arm or leg, slurred speech, chest pain, sudden breathlessness, a painful swollen leg, or severe belly pain. Tell them the person has polycythaemia vera or essential thrombocythaemia. Do not wait to read further, and do not wait for a clinic appointment.
The short answer
How is thrombosis risk scored in an MPN?
Your haematologist sorts your clot risk using a few simple facts: your age, whether you have ever had a clot, and in essential thrombocythaemia your gene result. Being over 60 or having had a clot before puts you in the higher-risk group, and that group usually needs treatment to bring counts down.
Why thrombosis matters so much
Thrombosis means a blood clot forming inside a blood vessel. In polycythaemia vera and essential thrombocythaemia, both types of myeloproliferative neoplasm (MPN), clots are the most common serious problem. They can affect the leg veins, lungs, heart, brain, or the veins of the liver and gut. Preventing them is the first aim of treatment.
A score, not a prediction
The risk score is a sorting tool built from studies of large groups of patients. It tells your doctor how much protection you probably need. It does not tell you whether you personally will have a clot, or when. Two people in the same group can have very different paths.
What your haematologist is really asking
The question behind the score is simple. Does the benefit of lowering your counts with a daily medicine outweigh its side effects for you? In the lower-risk group the answer is often no, and careful monitoring is enough. In the higher-risk group the answer is usually yes. Ask which side of that line you are on, and why.
Not sure whether this applies to you?
Ask an oncologistThe groups
Which risk groups are used for polycythaemia vera and ET?
The two conditions use slightly different systems. Find the one that matches your diagnosis.
Polycythaemia vera: low risk
Aged 60 or under and never had a clot. You still need your haematocrit controlled, usually with venesection, and most people take low-dose aspirin.
Polycythaemia vera: high risk
Over 60, or a clot in the past, or both. Along with venesection and aspirin, a medicine to lower the counts is usually added.
ET: very low and low risk
Aged 60 or under with no past clot. Very low risk means no JAK2 gene change. Low risk means a JAK2 change is present, which raises the chance of a clot.
Usual approach
- Monitoring, with or without aspirin
- Control of blood pressure, sugar and smoking
ET: intermediate and high risk
Intermediate means over 60 with no JAK2 change and no past clot. High risk means a past clot, or being over 60 with a JAK2 change.
Usual approach
- Aspirin considered
- Count-lowering medicine in the high-risk group
Step by step
How does your haematologist work out your group?
Confirm the diagnosis
Blood counts, gene tests and sometimes a bone marrow test show whether this is polycythaemia vera, ET or a related condition. The scoring system depends on which one it is.
Ask about past clots
Any clot counts, including an old leg clot, a mini-stroke, a heart attack or a clot during pregnancy. Bring discharge summaries if you have them.
Note your age and gene result
Age above 60 raises the group. In ET, a JAK2 change raises it further. A CALR or MPL change carries a lower clot risk.
Add your heart and vessel health
Smoking, high blood pressure, diabetes and high cholesterol are not part of the formal score, but they change the treatment decision.
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In practice
What does your group change about treatment?
Lower-risk group
Higher-risk group
Regular blood tests and clinic reviews
Regular reviews, often more frequent at first
Aspirin for many, especially in polycythaemia vera
Aspirin, or a stronger blood thinner after some clots
Count-lowering medicine usually not needed
Count-lowering medicine such as hydroxyurea or interferon usually added
Group can change with age or a new clot
Group does not usually move back down
Commonly believed
What do people misunderstand about clot risk scores?
"Low risk means I will never get a clot."
Low risk means lower, not none. You still need your counts watched, your blood pressure and sugar controlled, and you still need to know the warning signs of a clot.
"High risk means a clot is coming soon."
High risk means you gain more from treatment. With counts controlled and other risks managed, many people in this group go many years without a clot.
"The higher my platelet count, the higher my clot risk."
In ET the platelet number on its own is a weaker guide than age, past clots and the gene result. A very high count can even raise the risk of bleeding rather than clotting.
"Smoking does not matter because the problem is in my blood."
Smoking damages blood vessels and adds to MPN clot risk. Stopping smoking is one of the most useful things you can do, and help to quit is available.
Being straight with you
What can a risk score not tell you?
A score cannot see everything. It does not include your weight, your activity, long journeys, surgery, pregnancy or dehydration, all of which raise clot risk for a while. Your haematologist weighs those alongside the formal group.
It is not a measure of how long you will live
Clot risk groups are about preventing clots. They are not a survival score. Separate tools exist to judge how an MPN may change over time, and those are explained to you by your haematologist, not read off a chart.
Your group can change
Turning 60, a new clot or a change in your gene result moves you into a different group. That is why the question is asked again at reviews. Keep a list of every clot, procedure and hospital stay so the history is accurate each time. If an older family member is being looked after by you, keep this list on your phone so it is ready at every visit, and at any emergency department.
Never start, stop or change aspirin, a blood thinner or a count-lowering tablet because of what a score suggests. Discuss it with the team treating you.Questions we are asked
Common questions about MPN clot risk
What is the IPSET-thrombosis score?
It is the scoring system most often used for clot risk in essential thrombocythaemia. The revised version uses three facts: your age, whether you have had a clot, and whether you carry a JAK2 gene change. Together they sort you into very low, low, intermediate or high risk.
Why is age 60 used as the cut-off?
Large studies found that clot rates in MPNs rise clearly in people older than this. It is a practical line, not a switch that flips on a birthday. Your haematologist may act a little earlier if you have other risks such as diabetes, high blood pressure or a strong family history of clots.
Does a clot in a family member count?
It is not part of the formal score, but tell your haematologist. A strong family history of clots can point to an inherited clotting tendency, which may be tested for and can change advice about pregnancy, surgery, the contraceptive pill and long-distance travel.
I had a clot years before my MPN was found. Does that count?
Usually yes. Many people have a clot as the first sign of an MPN, often long before the diagnosis. A past clot of any kind places you in the higher-risk group. Share the date, the place of the clot and any hospital papers so your haematologist can judge it properly.
Can my risk group go down with treatment?
Treatment lowers your actual chance of a clot, but the group itself is based on age and history, which do not reverse. That is why people in the higher-risk group usually stay on their treatment long term, with regular reviews to check it still suits them.
Are clots in unusual places more common in MPN?
Yes. MPNs are linked with clots in the veins of the liver, bowel and brain, which are rare in other people. Severe belly pain, a swelling belly, yellow eyes or a severe new headache need urgent assessment. Go to emergency or call 108 rather than waiting.
What can I do myself to lower my clot risk?
Stop smoking, keep blood pressure, sugar and cholesterol controlled, stay active, and drink enough fluid, especially in hot weather and when travelling. Keep every blood test and clinic appointment. Take your medicines exactly as prescribed, and tell every doctor you see that you have an MPN.
Is the risk score the same as the JAK2 test?
No. The JAK2 test is one piece of information that feeds into the score in ET. The score combines it with your age and clot history. A JAK2 result on its own does not tell you your group, and it does not tell you how the disease will behave.
Your Haematologist
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Dr. Basudev Pokhrel reviews blood counts, transfusion needs and blood disorders, and works with the CION tumour board on blood cancers.
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Sources
- NHS — Polycythaemia
- NHS — Essential thrombocythaemia
- Leukemia & Lymphoma Society — Essential thrombocythemia
- NHLBI — Polycythemia Vera
- NHS — Stroke symptoms
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
Aspirin in MPN and Why It Is GivenWho is offered aspirin in each risk groupHydroxyurea in MPN: What to ExpectThe count-lowering medicine used in higher-risk groupsJAK2 Mutation Testing: What a Positive MeansHow the gene result feeds into your scoreSurgery and Travel With an MPNTimes when clot risk rises for a while
Talk to us
Not sure which risk group you are in?
Share your reports and clot history. CION's haematology team will explain your group and what it means for your treatment. One helpline serves every CION centre.