What is polycythaemia vera?

Polycythaemia vera (PV) is pronounced pol-ee-sigh-thee-me-a vee-rah. Poly means many and cythaemia relates to blood cells. It is also sometimes called erythrocytosis, which means too many red blood cells.

PV is a type of rare blood disorder called a myeloproliferative neoplasm. These are conditions that cause an increase in the number of blood cells.

The World Health Organisation (WHO) classes all myeloproliferative neoplasms as blood cancers. This is because the bone marrow is producing blood cells in an uncontrolled way. But many people with myeloproliferative neoplasms feel well and only need gentle treatment. The disorders often develop slowly and progress slowly. Or they can remain stable for a while.

Polycythaemia vera usually develops very slowly. It can affect people of any age. But it usually affects middle aged and older people. The average age at diagnosis is 60 years, although it can affect children and young adults.

The bone marrow and blood cells

Blood cells are made in the soft inner part of the bones, the bone marrow. All blood cells start from the same type of cell called a blood stem cell. The stem cell makes immature blood cells.

The immature cells go through various stages of development before they become fully developed blood cells and are released into the blood as:

  • red blood cells to carry oxygen
  • white blood cells to fight infection
  • platelets to help the blood clot

The diagram shows how the different types of cells develop from a single blood stem cell.

A simplified diagram showing how blood cells are made

What happens in polycythaemia vera?

In polycythaemia vera, the stem cells make too many red blood cells. This makes the blood become thicker. Sometimes the extra cells collect in the spleen, which can make the spleen become enlarged.

For many people, treatment can control PV for many years. But for some people, PV can lead to other problems. 

Some people develop scarring of their bone marrow over time. This is called myelofibrosis. And less commonly, people with PV can develop acute myeloid leukaemia (AML).

Secondary polycythaemia

Having too many red blood cells in the blood is not always due to changes in the bone marrow. Lung disease and kidney disease can also lead to an increase in red blood cells. This is called secondary polycythaemia. Secondary polycythaemia can also develop if you smoke.

The treatment for secondary polycythaemia is different to PV. You will have treatment for the underlying condition that caused the increase in blood cells. And your healthcare team might discuss how lifestyle changes could help.

Risks and causes

We know from research that more than 95 out of 100 people (more than 95%) who have polycythaemia vera have a change in the JAK2 gene Open a glossary item.

The JAK2 gene makes a protein that controls how many blood cells the stem cells make. A fault with your JAK2 gene, means the stem cells can start producing red blood cells when they're not meant to. This results in too many red cells being produced.

In most cases, these genetic faults happen during a person’s lifetime. You are not born with them, so you can’t pass them onto your children.

Family history

In rarer cases, you might have a history of myeloproliferative neoplasms (MPNs) in your family. This might mean there is a faulty gene in your family that increases your risk of developing MPNs.

Your specialist can talk with you about any genetic changes and family risk of MPNs. Ask if you have any questions or if this is worrying you.


Many people find out they have PV when they are having a blood test for something else. This is because PV usually develops slowly and doesn’t cause symptoms at first. As it progresses it causes symptoms. These include:

  • headaches
  • tiredness (fatigue)
  • burning pain in the hands and feet
  • blurred vision and other problems with your eyes, such as vision loss that comes and goes
  • night sweats
  • reddening of the skin
  • itching, especially after a shower or bath, this can be severe - doctors call this pruritus
  • pain or discomfort in the tummy (abdomen) from an enlarged spleen
  • high blood pressure
  • gout - you might have painful, stiff or swollen joints
  • bleeding or blood clots, which can sometimes lead to stroke or heart problems

Polycythaemia is rare, so if you have any of these symptoms it is more likely to be something else. But it is still important to contact your doctor to find out what is causing them.

  • Management of polycythaemia vera: a critical review of current data

    MF McMullin, BS Wilkins and CN Harrison

    British Journal of Haematology, 2016. Volume 172, Issue 3

  • The 2016 revision to the World Health Organization classification of myeloid neoplasms and acute leukemia

    D A Arber and others

    Blood, 2016. Volume 127, Issue 20

  • Clinical manifestations and diagnosis of polycythemia vera

    UpToDate Accessed April 2020

  • A guideline for the diagnosis and management of polycythaemia vera. A British Society for Haematology Guideline

    MF McMullin and others

    British Journal of Haematology, 2019. Volume 184, Issue 2

  • Blast Transformation in Myeloproliferative Neoplasms: Risk Factors, Biological Findings, and Targeted Therapeutic Options

    A Lurlo, D Cattaneo and U Gianelli

    International Journal of Molecular Sciences, 2019. Volume 20, Issue 8

  • Wintrobe’s Atlas of Clinical Hematology, 2nd edition

    BB Weksler, GP Schechter and S Ely

    Wolters Kluwer, 2018

Last reviewed: 
11 May 2020
Next review due: 
11 May 2023

Related links