Polycythemia Vera (PV): Understanding Your Pathology Report

Section Editor: David Li MD
June 24, 2026

Polycythemia vera (PV) is a type of blood cancer called a myeloproliferative neoplasm. It develops when the bone marrow produces too many red blood cells, and many people also have increased white blood cell and platelet counts. In most cases, polycythemia vera is caused by an acquired genetic change in a gene called JAK2 that sends constant signals telling blood cells to grow even when the body does not need them. The disease usually develops slowly and is often discovered after abnormal blood test results or during the investigation of symptoms or an unexpected blood clot.

This article will help you understand the findings in your pathology report for polycythemia vera, what each term means, and why it matters for your care.

Where is polycythemia vera found?

Polycythemia vera (PV) affects the blood and bone marrow. Over time, excess blood cells can also collect in the spleen and, less commonly, the liver, causing these organs to enlarge.

What are the symptoms of polycythemia vera?

The symptoms of polycythemia vera (PV) vary widely. Some people have no symptoms and are diagnosed after routine blood tests show high red blood cell levels. Others develop symptoms related to thicker blood and increased blood cell counts. Common symptoms include fatigue, headaches, dizziness, difficulty concentrating, night sweats, and itching. The itching is often triggered or worsened by warm water, such as during a bath or shower. Some people develop redness of the skin, especially the face, called plethora, and an enlarged spleen can cause discomfort or a feeling of fullness in the upper-left abdomen.

Polycythemia vera increases the risk of blood clots. In about one in five people, the first sign of the disease is an unexpected clot, such as a clot in the deep veins of the leg, the lungs, the brain (stroke), or the heart (heart attack). Clots in the veins of the abdomen are particularly suggestive of an underlying myeloproliferative neoplasm, even when blood counts appear normal at the time. Bleeding can also occur, although it is less common than clotting. Iron deficiency, from repeated blood removal or bleeding, can sometimes mask the true extent of the increase in red blood cell count.

How common is polycythemia vera?

Polycythemia vera (PV) is uncommon. Large population studies estimate that about 1.5 to 1.6 people per 100,000 are diagnosed each year. It is most often found in older adults, although it can occur at any age.

What causes polycythemia vera?

The exact cause of polycythemia vera (PV) is unknown, and the disease is not usually inherited. In most people, it is driven by an acquired mutation in the JAK2 gene that develops during life. Rarely, prior exposure to high levels of radiation has been linked to an increased risk.

What genetic changes are involved in polycythemia vera?

Almost all people with polycythemia vera (PV) have a mutation in the JAK2 gene. The most common one, found in about 95% of cases, is called JAK2 V617F. A smaller number of people (about 3 to 4%) instead have a mutation in a part of the gene called exon 12. These mutations cause the JAK2 protein to remain continuously active, leading to increased production of red blood cells and, in some cases, other blood cells.

Additional mutations in other genes, such as TET2, ASXL1, IDH2, and SRSF2, may also be present. These are more common as the disease progresses and may influence the long-term outlook. Some people also have chromosome changes detected by cytogenetic testing, such as a deletion of part of chromosome 20 or extra copies of chromosomes 8 or 9, which are more common in advanced disease. A dedicated article explains JAK2 testing in more detail: JAK2 Mutations in Myeloproliferative Neoplasms.

What are the disease phases of polycythemia vera?

Polycythemia vera (PV) usually begins as a chronic disease that can remain stable for many years. In the chronic phase, red blood cell production increases, white blood cell and platelet counts may also be elevated, and symptoms are often related to thickened blood or increased cell mass. Over time, some people develop more advanced disease:

Post-polycythemic myelofibrosis — About 20% of people eventually develop scarring of the bone marrow, which makes it less able to produce normal blood cells. This can lead to anemia, low platelet counts, fatigue, and significant enlargement of the spleen.
Transformation to acute leukemia — In a smaller number of people, polycythemia vera can progress to acute myeloid leukemia, a serious complication with a much less favorable outlook.

How is the diagnosis made?

The diagnosis of polycythemia vera (PV) is based on blood tests, a bone marrow examination, and molecular testing, used together to confirm the diagnosis and rule out other causes of a high red blood cell count. Blood tests typically show an increased red blood cell count, hemoglobin level, and hematocrit (the percentage of the blood made up of red blood cells). White blood cells, especially neutrophils, and platelets may also be increased. A blood test for erythropoietin, the hormone that signals the body to make red blood cells, usually shows a low level in polycythemia vera, which is an important clue. On a blood smear, the red blood cells may appear crowded, and in advanced disease, particularly post-polycythemic myelofibrosis, teardrop-shaped red blood cells may be seen.

A bone marrow biopsy is often performed to support the diagnosis and assess the stage. In untreated polycythemia vera, the bone marrow is usually hypercellular, meaning it contains more blood-forming cells than expected, and all three major blood cell lines are increased, a pattern called panmyelosis. The cells that make platelets, called megakaryocytes, are increased and abnormal, often varying in size with large, deeply folded nuclei, and may form loose clusters. There is usually little scarring at the time of diagnosis, and iron stores are typically reduced. In post-polycythemic myelofibrosis, the bone marrow shows moderate-to-severe scarring.

Molecular testing for the JAK2 mutation, described above, is a key part of confirming the diagnosis and may be performed on blood or bone marrow as part of a larger molecular testing panel. Polycythemia vera must be separated from secondary causes of a high red blood cell count, such as chronic lung or heart disease, smoking, living at high altitude, or rare tumors that produce erythropoietin. A low erythropoietin level together with a JAK2 mutation strongly supports polycythemia vera. It must also be distinguished from other myeloproliferative neoplasms, such as essential thrombocythemia and prefibrotic primary myelofibrosis, which can have overlapping features.

What happens after a diagnosis of polycythemia vera?

After polycythemia vera (PV) is diagnosed, treatment aims to relieve symptoms and lower the risk of blood clots and disease progression. The care team first estimates the risk of clotting, which is considered higher in people aged 60 or older and in anyone who has already had a blood clot. The pathology and blood findings help guide several decisions:

Phlebotomy — Removing blood at intervals to lower the hematocrit and keep it below 45%, which reduces the risk of clots. Iron supplements are generally avoided.
Low-dose aspirin — Often used to further lower the risk of clotting, unless there is a reason to avoid it.
Cytoreductive therapy — For higher-risk disease, or when phlebotomy and aspirin are not enough, medicines that lower blood cell production may be added. Options include hydroxyurea, interferon (such as ropeginterferon alfa-2b), and the JAK inhibitor ruxolitinib, which is often used when hydroxyurea is not tolerated or no longer works.
Managing symptoms and other risks — Controlling itching and addressing cardiovascular risk factors such as blood pressure, cholesterol, and smoking.

Follow-up includes regular blood tests and clinical assessments to monitor treatment response and watch for signs of progression, such as increasing bone marrow scarring or changes in blood counts. Newer treatments are being studied in clinical trials, which may be an option to discuss. Decisions about treatment are made by the care team together with the patient, based on the specific findings in the report.

What is the prognosis for polycythemia vera?

Polycythemia vera (PV) is generally a slow-growing disease, but it does shorten life expectancy compared with the general population. The main risks are blood clots, bleeding, progression to myelofibrosis, and transformation to acute leukemia. The risk of leukemia increases over time and is influenced by factors such as age, high white blood cell counts, and specific genetic changes. With careful monitoring and appropriate therapy, many people live for years or decades. Your prognosis depends on your own combination of these factors, which your care team can explain in the context of your specific report.

Questions to ask your doctor

What features of my blood tests support a diagnosis of polycythemia vera?
Was a JAK2 mutation found, and which type do I have?
Was my erythropoietin level measured, and was it low?
Do my bone marrow biopsy results show any scarring?
Am I considered low risk or high risk for blood clots?
What hematocrit level are we aiming for, and how often will I need phlebotomy?
Should I be taking low-dose aspirin?
Do I need a medication to lower my blood counts, and if so, which one?
What can be done about the itching?
What signs might suggest progression to myelofibrosis or leukemia?
Are there clinical trials that I should consider?