Updated 09 Sep 2019

If you've got polycythaemia vera (PV), too many red blood cells are made in your bone marrow. Your chance of getting PV increases with age – around 95% of people with PV are aged 40 years or older.

What is polycythaemia vera (PV)?

PV is a myeloproliferative neoplasm. If you have it, you will have too many red blood cells in your bone marrow. As a result, your blood becomes thicker than normal. Some people with PV may also have increased numbers of white cells and platelets.

What causes PV?

All cells in your body contain a set of instructions which tell the cell what to do and when to do it, stored inside the cells in structures called chromosomes. The chromosomes are made up of a chemical known as DNA.

The DNA is arranged in sections called genes. There are 23 pairs of chromosomes in each cell in your body. When cells divide to form new cells, normally the chromosomes stay the same in each new cell.

However with PV, something goes wrong and causes a genetic fault to occur: you may hear your doctor talk about a fault, or mutation, in the JAK2 gene. This may happen because you’ve been exposed to hazardous chemicals, but more usually because of copying mistake when a cell was dividing. Around 95% of people with PV have this genetic fault.

The JAK2 gene is involved in the response of bone marrow stem cells to different growth factors. A growth factor is a substance which sends signals to your stem cells, so they can produce the right number of blood cells to keep you healthy.

When you have a fault with your JAK2 gene, the stem cells can start producing red blood cells even when they’ve not be ‘told’ to do so by growth factors. This results in too many red cells being produced.

It’s important to note that the genetic fault happens during a person’s lifetime. As you’re not born with it, you can’t pass it onto your children.

As well as the presence of certain genetic faults, there are other factors which might lead to a higher risk of getting PV.


Your chance of getting PV increases with age. Around 95% of people diagnosed with PV are aged 40 years or older. The disease is rare in people under the age of 15 years.


PV is slightly more common in men than women; we’re not sure why.

Symptoms and diagnosis

You may not have any symptoms at all before or when you’re diagnosed. That’s why many people with PV are diagnosed following a routine blood test.

The increase in red blood cells makes it hard for blood to flow smoothly through your blood vessels. This is known as hyperviscosity and may mean you get some of these symptoms:

  • headaches,
  • confusion,
  • blurred vision,
  • skin reddening (plethora), and
  • itchy skin (pruritus).

A less common complication of PV is gout, which can cause inflammation of the joints.


People with PV are at an increased risk of thrombosis (blood clots). Thrombosis is a serious condition and may occur in the blood vessels of your:

  • brain (causing a stroke or mini-stroke/TIA – transient ischaemic attack),
  • eyes (causing blurred vision or loss of vision), and
  • heart (causing a heart attack).

Blood clots can also form in the veins of your legs. This is known as deep vein thrombosis or DVT. Clots can also form in the vessels in your abdomen (stomach area). If a clot dislodges and travels to the lung, it may cause a pulmonary embolism (or PE). This usually results in low oxygen levels, sharp chest pain and shortness of breath. In some cases this may be fatal. The risk of clots is highest if the PV isn’t treated.

You’ll have regular blood tests so your healthcare team can monitor your condition and spot any early signs of a blood clot.

Symptoms of a blood clot:

  • sudden chest pain or shortness of breath,
  • swelling and/or pain in your calf on one side,
  • slurred or abnormal speech, weakness in your arms or legs, or drooping on one side of your face,
  • swelling in your abdomen or jaundice (your skin turning a yellow colour), and
  • sudden loss of vision in one eye.

If you have any of these symptoms, you should get urgent medical attention.

Tests for PV

Most people are suspected of having PV after a routine blood test or by going to their GP with symptoms. You’d then have a set of tests to confirm the exact diagnosis of PV.

Full blood count

Polycythaemia (rather than polycythaemia vera) is defined as a persistent increase in the proportion of your blood that’s made up of red blood cells. Your doctor may refer to this as your ‘packed cell volume’ (or PCV). This is usually checked using a test called a full blood count.

For this test a small sample of your blood will be taken, then the cells will be studied under a microscope in a laboratory.

Bone marrow biopsy

Some people need tests on their bone marrow done before their doctors can make a diagnosis. Before these tests can be carried out, doctors will need to take a bone marrow sample (biopsy). This helps to rule out any other bone marrow problems.

The result of these tests alone aren’t enough for your doctor to diagnose PV, as there are a number of other reasons and conditions that can mean you have too many red blood cells.

These include:

  • smoking,
  • lung diseases,
  • sleep apnoea (where your normal breathing is disrupted while you’re asleep),
  • living at high altitudes,
  • some kinds of tumours,
  • anabolic steroids (prescription performance-enhancing drugs),
  • testosterone treatment, and
  • some rare inherited genetic disorders.

JAK2 blood test

The discovery of the JAK2 genetic fault in 2005 has made it easier to diagnose PV. Some of your DNA will be taken from a sample of your blood, and tested. If you have this genetic fault, doctors will be able to confirm you have PV.

Further tests

If there’s no clear cause for your polycythaemia and you don’t have the JAK2 genetic fault, your doctor will do more tests to confirm a diagnosis.

These may include:

  • further blood tests for erythropoietin (a hormone that increases red blood cell production) levels and other genetic tests,
  • red cell mass studies (using radioisotopes) to distinguish ‘apparent’ polycythaemia from actual polycythaemia,
  • tests on blood samples taken from an artery (instead of vein) to measure your oxygen levels,
  • tests on samples of your bone marrow,
  • lung function tests, and
  • scans to see if your spleen is swollen or other possible causes – for example, a tumour which is releasing erythropoietin.

Your doctor will be able to talk to you about any of these tests and explain how they’re done and what they’re looking for. Be sure to ask them any questions about these tests if you’re unsure.

Treatment and side effects

The aim of your treatment is to reduce the risk of getting thrombosis by reducing the number of red blood cells in your blood. You should also be monitored for cardiovascular risk factors such as diabetes, high cholesterol, high blood pressure and smoking, and these should be addressed as effectively as possible.

Although currently PV can’t be cured, it can be kept under control to reduce the symptoms and complications it may cause.

The treatment you receive for PV will depend on the following factors:

  • how high the packed cell volume (PCV) is,
  • your age, and
  • the type of blood cell most affected.

If you’ve been diagnosed with PV and don’t have any symptoms, you may not need to start treatment for a while. Instead, you'll be monitored with regular check-ups and blood tests; this is known as 'watch and wait'. While this might seem strange, there’s no evidence to show that treating people with no symptoms has any impact on their outcome. It also means you don’t get any side effects from unnecessary treatment.

If you do have symptoms, your treatment will include some or all of the following options.


This is one of the simplest and quickest ways to reduce the number of red blood cells in your blood and make your blood thinner. This is also known as blood-letting or phlebotomy. It involves taking around a pint (half a litre) of blood from you. This may be done once a week initially and then repeated as often as needed.

You may feel faint after the blood is taken, so replacement fluid can be given at the same time to help with this.

Low-dose aspirin

If you have PV and other risk factors such as previous clots, diabetes and high blood pressure, you may be at an increased risk of blood clots so your doctor may recommend you take low-dose aspirin regularly. If you need to take painkillers for any other reason at the same time, ask your doctor what you can safely take with the aspirin. If you are already taking an anticoagulant such as warfarin, your doctor may decide you do not need aspirin as well.


If your platelet count is high or you have other symptoms such as weight loss or sweats, you may be given tablets called hydroxycarbamide (or hydroxyurea) to take. This is a mild form of chemotherapy and works by directly preventing the production of red blood cells. Hydroxycarbamide is the most common chemotherapy drug used to treat PV.

You might get some side effects from this treatment. These might include more infections than normal, diarrhoea or constipation. Your healthcare team will be able to help you manage side effects like this.

Hydroxycarbamide is a very safe treatment. However, there’s a theoretical risk that it may increase the risk of PV transforming into acute myeloid leukaemia if it’s used as a long term treatment. For many people, the benefits of the treatment usually outweigh any potential small risk.


Interferon is another drug you may take if your platelet count is high or you have symptoms such as weight loss or sweating. Interferon is an injection that reduces the rate at which blood cells are made. It’s not thought to carry the same risk of developing leukaemia as hydroxycarbamide when used in the long-term, and is the preferred choice for younger patients. However, many people find the short-term side effects unpleasant and not everyone can tolerate interferon therapy.

Possible side effects you may get while being treated with interferon include:

  • flu-like symptoms,
  • headaches,
  • dizziness,
  • mood swings, and
  • tiredness.


Ruxolitinib is a type of drug known as a JAK2 inhibitor. It can sometimes be used for people with PV when hydroxycarbamide is not working well, but it is not routinely available.


Busulfan is another chemotherapy drug. It may be used when hydroxycarbamide isn’t appropriate or isn’t working. It’s given as a tablet. Side effects can include lung tissue damage or reduced numbers of red blood cells, white blood cells or platelets in the blood. Busulfan can also increase the risk of leukaemia developing, so it’s only used when doctors believe the benefits of treatment outweigh the risks.


As PV is generally diagnosed in later life, there’s a very good chance that people who are diagnosed with it will have a normal lifespan and a good quality of life if the condition is carefully monitored and treated as needed.

Around 5% of people with PV go on to develop a more aggressive cancer. This may include progressing to myelofibrosis, where the bone marrow becomes scarred and less able to produce cells, or to acute myeloid leukaemia.

You may find it hard to ask or talk about your prognosis. Sometimes those close to you might want to know your prognosis even if you don’t. However, your healthcare team aren’t allowed to give this or any other information to anyone – not even family members – without your permission. Try to decide early on who you want to know about your condition, then tell your healthcare team – you can change your mind at any time.

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