Hematocrit (Hct) is the percentage of red blood cells (RBCs) in your blood. When hematocrit rises above the normal range—particularly in PV—blood viscosity can increase, which can affect circulation in small vessels. [1,2,3] This is one reason clinicians focus on hematocrit control in PV.

The CYTO-PV trial showed that targeting hematocrit <45% reduced the rate of cardiovascular death and major thrombosis compared with a target of 45–50% [1].

In PV, hematocrit can be high due to: [4,6]

1. Bone marrow overactivity: JAK2-driven signaling promotes increased red blood cell production.
2. Low erythropoietin (EPO): EPO may be suppressed because the body senses increased red cells and downshifts the usual feedback signal.
3. Increased red cell mass: Excess red cells raise hematocrit even when plasma volume is otherwise normal. [4]

When hematocrit or red blood cell counts are high, symptoms may include: [4,8]

• Headaches and dizziness
• Red or flushed face (plethora)
• Vision changes (blurred or double vision)
• Itching, especially after warm showers
• Numbness, tingling, or burning in hands and feet
• Fatigue and shortness of breath

These symptoms can be related to changes in blood flow/viscosity and microvascular circulation. [4,8]

A poorly controlled hematocrit is linked to clinically important complications, including:

• Blood clots (thrombosis): a major contributor to morbidity and mortality in PV.
• Heart attack or stroke: when clots affect arteries.
• Venous thrombosis: Including deep vein thrombosis (DVT) or pulmonary embolism (PE).
• Progression to myelofibrosis: in some patients over time.

Clinical studies support increased thrombotic risk when hematocrit is not well controlled. [9,10]

Hematocrit is part of a complete blood count (CBC) test. Clinicians often review related markers too: Hemoglobin (Hgb), Red blood cell (RBC) count and other blood counts (white cells / platelets) as clinically relevant in PV. [4,6]

Managing hematocrit is a cornerstone of PV care, and many treatment approaches aim for hematocrit <45%, based on randomized evidence. [4,6] PV hematocrit control usually relies on phlebotomy and, for some patients, medications that reduce blood cell overproduction (such as interferon-based therapy or hydroxyurea), chosen by the care team based on individual risk and response. [4,5,6]

If appropriate, low-dose aspirin may be used to lower clot risk. Healthy habits (avoid smoking, stay hydrated, be active as tolerated) can help reduce clot risk and support circulation as part of overall self-care in PV, and regular CBC checks help your team keep hematocrit in the intended range and adjust treatment. [4,5,6,11,12,13]

Contact your healthcare provider urgently if you experience:

• Sudden or severe headaches
• Vision changes or dizziness
• Chest pain or shortness of breath
• Swelling in legs or unexplained leg pain

These may reflect a thrombotic complication or another urgent issue. [4]

1. What does a high hematocrit mean?

It means red blood cells make up a higher-than-normal share of your blood (hematocrit is the percentage of blood occupied by red blood cells), which can increase blood thickness (hyperviscosity). [3,14] In PV, this occurs because PV is a clonal myeloproliferative neoplasm characterized by erythrocytosis (overproduction of red cells). [4]

2. Why is controlling hematocrit important in PV?

Because stricter hematocrit control (targeting <45%) reduced cardiovascular death and major thrombosis compared with less intensive control in a randomized trial. [9]

3. Can I lower hematocrit naturally?

Healthy habits (hydration, not smoking) are helpful for overall risk, but medical treatment is typically required to control hematocrit in PV. [4,5,6,13,15]

4. How often should hematocrit be checked?

Testing frequency is individualized. It is often checked more frequently during treatment adjustments and at routine follow-up thereafter.[12]

5. What treatments help reduce high hematocrit?

Common approaches include phlebotomy, low-dose aspirin (when appropriate), and cytoreductive therapy for selected patients. [4,6]

PV — Polycythemia vera

Hct — Hematocrit

RBCs / RBC — Red blood cells / Red blood cell

JAK2 — Janus kinase 2

EPO — Erythropoietin

DVT — Deep vein thrombosis

PE — Pulmonary embolism

CBC — Complete blood count

Hgb — Hemoglobin

CYTO-PV — CYTO-PV trial

FAQs — Frequently asked questions

1. National Library of Medicine. (n.d.). Hematocrit test. MedlinePlus. https://medlineplus.gov/lab-tests/hematocrit-test/
2. Kishimoto, S., Maruhashi, T., Kajikawa, M., et al. (2020). Hematocrit, hemoglobin and red blood cells are associated with vascular function and vascular structure in men. Scientific Reports, 10, 11467. https://doi.org/10.1038/s41598-020-68319-1
3. StatPearls Publishing. (2025). Polycythemia. In StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK526081/
4. Tremblay, D., Kremyanskaya, M., Mascarenhas, J., & Hoffman, R. (2025). Diagnosis and treatment of polycythemia vera: A review. JAMA, 333(2), 153–160. https://doi.org/10.1001/jama.2024.20377
5. Barbui, T., Tefferi, A., Vannucchi, A. M., Passamonti, F., Silver, R. T., Hoffman, R., … Barosi, G. (2018). Philadelphia chromosome-negative classical myeloproliferative neoplasms: Revised management recommendations from European LeukemiaNet. Leukemia, 32(5), 1057–1069. https://doi.org/10.1038/s41375-018-0077-1
6. Tefferi, A., & Barbui, T. (2024). Polycythemia vera: 2024 update on diagnosis, risk-stratification, and management. American Journal of Hematology. https://doi.org/10.1002/ajh.27002
7. Waggoner, M. (2023). Polycythemia vera: Thinking beyond the hematocrit. Journal of the Advanced Practitioner in Oncology, 14(5), 405–413. https://doi.org/10.6004/jadpro.2023.14.5.5
8. Marchioli, R., Finazzi, G., Specchia, G., et al. (2013). Cardiovascular events and intensity of treatment in polycythemia vera. New England Journal of Medicine, 368(1), 22–33. https://doi.org/10.1056/NEJMoa1208500
9. Kuykendall, A. T., Fine, J. T., & Kremyanskaya, M. (2024). Contemporary challenges in polycythemia vera management from the perspective of patients and physicians. Clinical Lymphoma Myeloma and Leukemia, 24(8), 512–522. https://doi.org/10.1016/j.clml.2024.04.003
10. Landolfi, R., Marchioli, R., Kutti, J., et al. (2004). Efficacy and safety of low-dose aspirin in polycythemia vera. New England Journal of Medicine, 350(2), 114–124. https://doi.org/10.1056/NEJMoa035572
11. Alberta Health Services. (2025). Polycythemia vera (PV): Clinical practice guideline LYHE-009. https://www.albertahealthservices.ca/assets/info/hp/cancer/if-hp-cancer-guide-lyhe009-pv.pdf
12. Blood Cancer UK. (n.d.). Looking after yourself with PV. https://bloodcancer.org.uk/understanding-blood-cancer/polycythaemia-vera-pv/looking-after-yourself-with-pv/
13. National Cancer Institute. (n.d.). Definition of hematocrit. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/hematocrit
14. Waggoner, M. (2023). Polycythemia vera: Thinking beyond the hematocrit. Journal of the Advanced Practitioner in Oncology, 14(5), 405–413. https://pmc.ncbi.nlm.nih.gov/articles/PMC10414534/
15. Benevolo, G., Marchetti, M., Melchio, R., Beggiato, E., Sartori, C., Biolé, C. A., Rapezzi, D., Bruno, B., & Milan, A. (2023). Diagnosis and management of cardiovascular risk in patients with polycythemia vera. Vascular Health and Risk Management, 19, 765–778. https://doi.org/10.2147/VHRM.S429995