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Polycythaemia means having a high concentration of red blood cells in your blood. This makes the blood thick and less able to circulate through the vessels to the organs.

Mild cases of polycythaemia may not cause any symptoms. However, many people with polycythaemia experience:

  • headaches
  • blurred vision
  • a ruddy complexion, due to the high number of red blood cells 
  • chest pain
  • high blood pressure
  • pain in the calf muscles when walking
  • dizziness
  • tinnitus (ringing in the ears)
  • periods of confusion
  • itchy skin, especially after a bath or shower – this is the result of white blood cells (levels of which can also be high) releasing the chemical histamine 

Many of the above symptoms are the result of a sluggish blood flow, and not enough oxygen getting to vital organs and tissues.

The reduced blood flow can also cause complications such as blood clots, bleeding problems (such as nosebleeds and bruising) and gout. Blood clots are particularly dangerous as they can put you at risk of a heart attackpulmonary embolism (blockage in the lungs) or stroke.

How it is diagnosed

Polycythaemia is diagnosed after a red blood cell count (blood test) shows that your blood is highly concentrated with red blood cells. Sometimes there is also a high concentration of white blood cells and platelets. 

Your GP may have ordered the blood test because you reported some of the above symptoms or complications. Or, polycythaemia may have been discovered during a routine blood test for another reason.

Your GP may refer you to a haematologist (blood specialist) for further tests to confirm the diagnosis and to determine the underlying cause.

Why it happens

There are two possible reasons why blood becomes concentrated with red blood cells:

  • Your red cell count is normal but you have a reduced amount of plasma (fluid) in your blood. This is known as 'relative polycythaemia'. The condition may settle if certain factors causing this disruption are changed – causes include excess weight, smoking, excess alcohol and some blood pressure medicines. 
  • You are producing too many red blood cells – a high red blood cell count is known as erythrocytosis, and there are two forms of this:

    polycythaemia rubra vera (PRV), where there is an abnormality within the red blood cells

    secondary polycythaemia, an overproduction of red blood cells resulting from another disease

Both PRV and secondary polycythaemia are explored in more detail below.

Sometimes, the cause of polycythaemia is unknown.

Polycythaemia rubra vera (PRV)

PRV is caused by a rare abnormality in a gene, which causes the bone marrow cells to multiply out of control and produce too many red blood cells.

People with PRV may also have abnormally high numbers of platelets (thrombocytosis) and abnormally high numbers of white bloods cells (leukocytosis).

Most people with PRV are diagnosed later in life, at an average age of 60.

Secondary polycythaemia

People with secondary polycythaemia have a condition that stimulates the bone marrow to produce more red blood cells than are needed. The types of diseases that cause polycythaemia are:

  • those causing low levels of oxygen in the blood, such as chronic obstructive pulmonary disease (COPD), or
  • a tumour or other disease that causes more erythropoietin to be produced – erythropoietin is the hormone that regulates the production of red blood cells

Treating polycythaemia

Treatment aims to prevent symptoms and complications (such as blood clots) and treat any underlying condition causing the polycythaemia.

Venesection (bloodletting) 

Venesection is the simplest and quickest way of reducing the number of red cells in your blood. About one pint (half a litre) of blood is removed at a time. It is similar to the procedure used for donating blood.

How often this is required will be different for each person. At first you may need the treatment every week. Once your condition is under control you may only need it every six-12 weeks. It may even be needed less often.

Your hospital stay should be no longer than one hour.

Drugs to slow the production of red blood cells

Medication can be prescribed to slow down the production of red blood cells, and to reduce the production of platelets if necessary.

Many different drugs are available, and your specialist will take into account your age and health, response to venesection and red blood cell count when choosing the most appropriate drug for you. Examples include:

  • hydroxycarbamide – generally tolerated well, but should not be taken by pregnant women or women trying to conceive
  • interferon – can be taken in pregnancy, but may cause unpleasant side effects such as hair loss and flu-like symptoms

Drugs to prevent blood clots

Low-dose aspirin may also be prescribed for you to take daily to help prevent blood clots. This is important, because a blood clot can become dislodged and travel through the circulation to cause a pulmonary embolismheart attack or stroke.

Treating and preventing other conditions

You may also need treatment for any symptoms or complications of polycythaemia, or for any underlying cause of polycythaemia. For example, antihistamines can be taken to help relieve itching.

You can read more about:

It's important to make any necessary lifestyle changes, such as quitting smoking and losing any excess weight, to lower your risk of a heart attack or stroke. Having polycythaemia means you are already at high risk of a blood clot, and being overweight or smoking will only increase this risk further.

You may find the following advice and information helpful:


If you do not have any other disease and do not develop any complications, polycythaemia should not affect your life expectancy. Any symptoms can usually be managed with medication so you can live a normal life.

The biggest risk to your health is developing a blood clot, and your likelihood of this increases with age. Read more about deep vein thrombosis, a clot that forms in one of the deep veins, usually in the leg. You should also be aware of the warning signs of a blood clot (see below).

Blood clot warning signs

  • pain or swelling in your leg
  • the skin of your leg feeling hot or looking discoloured
  • the veins near the surface of your leg appearing larger than normal
  • becoming short of breath
  • pain in your chest or upper back
  • coughing up blood

If you experience any of the above symptoms, get medical help immediately.

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