Several factors contribute to clot risk in PV:

1. Too many red blood cells: Increases blood thickness (viscosity), slowing circulation.
2. Excess platelets and white blood cells: These cells interact with vessel walls and promote clot formation [3].
3. Changes inside the blood vessels: PV can cause inflammation in blood vessels, making them “stickier” [4].
4. Other risk factors, such as older age, smoking, being overweight, high blood pressure, or a history of blood clots, can increase the likelihood of another clot [5].

In polycythemia vera, clots usually form in the following circulatory system:

• Deep vein thrombosis (DVT): A clot that forms in a deep vein, most commonly in the leg. Symptoms are usually localized to one leg and include pain, swelling, warmth, and redness.
• Pulmonary embolism (PE): A life-threatening emergency that can happen if a clot travels to the lungs. Warning signs include sudden shortness of breath, sharp chest pain, and a rapid heart rate.
• Stroke: Occurs when a clot blocks an artery supplying blood to the brain. Symptoms appear suddenly and require immediate medical attention: weakness or numbness on one side of the body, difficulty speaking, confusion, or a severe headache.
• Heart attack: Caused by a clot blocking an artery that feeds the heart muscle. This can result in chest pain or pressure, shortness of breath, and pain that radiates to the arm, back, or jaw.
• Unusual sites: Though rare, PV can cause serious clots in the veins of the liver (known as Budd-Chiari syndrome) or other abdominal organs, leading to severe pain and swelling [6].

Doctors often divide PV patients into “high risk” and “low risk” groups for clotting:

• High risk: Age 60 or older, or history of blood clots.
• Low risk: Younger patients with no history of clots.

Even younger people can develop blood clots; studies show that the risk remains significant for those under the age of 40 over several decades [7].

Treatment focuses on lowering blood counts and reducing clotting risk:

• Phlebotomy (blood removal): Keeps hematocrit (red cell concentration) below 45%, reducing viscosity [1].
• Aspirin: Low-dose aspirin helps prevent platelets (the tiny cells that help the blood clot) from sticking and forming clots [8].
• Cytoreductive therapy: Medications such as hydroxyurea or interferon are used to lower blood cell counts, which reduces the risk of clot events [9].
• Lifestyle changes, such as staying active, remaining hydrated, and avoiding smoking, also contribute to overall well-being.

Even with treatment, some patients may still develop clots. This is why regular check-ups and blood tests to monitor red cell levels and assess the effects of medication are critical. Doctors monitor levels of blood cells, symptoms, and factors related to heart and blood vessel health to adjust therapy.

With proper care, the risk of thrombosis can be significantly reduced, and patients with PV can live for many years [2].

1. Why does PV increase blood clot risk?

PV increases the number of red blood cells, making the blood thicker and less likely to flow quickly. Additionally, platelets and white blood cells may also be elevated, which increases the likelihood of blood clots forming within vessels. This combination creates a lifelong risk of thrombosis (clotting).

2. Are clots more common in men or women?

Both men and women with PV face a high risk of clotting. Some studies suggest men may have more arterial clots (like strokes or heart attacks), while women may more often develop clots in unusual sites, such as abdominal veins. Overall, both genders need the same close monitoring and prevention.

3. Can blood clots be prevented in PV?

Yes, prevention is a key part of PV treatment. Doctors use phlebotomy (regular blood removal) to lower red blood cell levels, aspirin to thin the blood, and medicines like hydroxyurea or interferon to control cell counts. Lifestyle steps, such as staying active, avoiding smoking, and managing blood pressure, also help reduce the risk of clot formation.

4. What symptoms should I watch for?

Warning signs of clots include leg swelling or pain (possible DVT), chest pain or shortness of breath (possible PE), sudden weakness or trouble speaking (possible stroke), and severe headaches or vision changes. Patients should seek urgent care if any of these symptoms appear.

5. Is the risk of clots lifelong in PV?

Yes, the clotting risk persists as long as the disease is present, but it can be managed. With treatment, the risk is significantly reduced, and many people live for decades after diagnosis without a major clotting event. Consistent follow-up and proper treatment are the most effective tools for ensuring long-term safety.

1. Stuart, B. J., & Viera, A. J. (2004). Polycythemia vera. Am Fam Physician, 69(9), 2139–2144.
2. Barbui, T., et al. (2014). Thrombosis rates in contemporary PV patients. Blood, 124(19), 3021–3023.
3. Kroll, M. H., Michaelis, L. C., & Verstovsek, S. (2015). Mechanisms of thrombogenesis in PV. Blood Rev, 29(4), 215–221.
4. Vannucchi, A.M. Insights into the pathogenesis and management of thrombosis in polycythemia vera and essential thrombocythemia. Intern Emerg Med 5, 177–184 (2010). https://doi.org/10.1007/s11739-009-0319-3
5. Griesshammer, M., Kiladjian, J. J., & Besses, C. (2019). Thromboembolic events in PV. Ann Hematol, 98(5), 1071–1082.
6. Landolfi, R., & Di Gennaro, L. (2008). Prevention of thrombosis in PV and ET. Haematologica, 93(3), 331–335.
7. Passamonti, F., et al. (2003). PV in young patients: risk of thrombosis, MF, leukemia. Haematologica, 88(1), 13–18.
8. Pearson, T. C. (2002, June). The risk of thrombosis in essential thrombocythemia and polycythemia vera. In Seminars in oncology (Vol. 29, No. 3, pp. 16-21). WB Saunders.
9. Tremblay, D., et al. (2024). Diagnosis and treatment of polycythemia vera: a review. JAMA.