Erythrocytosis, sometimes called polycythaemia, means having a high concentration of red blood cells in your blood.

This makes the blood thicker and less able to travel through blood vessels and organs. Many of the symptoms of erythrocytosis are caused by this sluggish flow of blood.

Symptoms of erythrocytosis

Not everyone with erythrocytosis has symptoms – but many do.

Make an appointment to see your GP if you have persistent symptoms of erythrocytosis. These include:

When to seek immediate medical advice

Erythrocytosis can cause blood clots. These put you at risk of life-threatening problems such as:

Seek medical help immediately if you or someone you're with shows signs of DVT or a pulmonary embolism. These include:

Erythrocytosis also increases your risk of heart attack and stroke. Seek emergency medical help if you think that you or someone you're with is having a heart attack or stroke.

What causes erythrocytosis?

Erythrocytosis can be divided into several different types, depending on the underlying cause. In some cases, an underlying cause can't be identified.

Apparent erythrocytosis

"Apparent erythrocytosis" is where your red cell count is normal, but you have a reduced amount of a fluid called plasma in your blood, making it thicker.

Apparent erythrocytosis is often caused by being overweight, smoking, drinking too much alcohol or taking certain medicines – including diuretics (tablets for high blood pressure that make you pee more).

Apparent erythrocytosis may improve if the underlying cause is identified and managed. Stopping smoking or reducing your alcohol intake, for example, may help.

Relative polycythaemia

This is similar to apparent erythrocytosis. It can happen as a result of dehydration.

Absolute polycythaemia

"Absolute erythrocytosis" is where your body produces too many red blood cells. There are 2 main types:

Primary erythrocytosis - polycythaemia vera

Polycythaemia vera is rare. It's usually caused by a change in the JAK2 gene, which causes the bone marrow cells to produce too many red blood cells. It’s a slow-growing type of blood cancer.

The affected bone marrow cells can also develop into other cells found in the blood, which means that people with polycythaemia vera may also have abnormally high numbers of both platelets and white bloods cells.

Although caused by a genetic change, polycythaemia vera isn't usually inherited. Most cases develop later in life. The average age at diagnosis is 60.

Secondary erythrocytosis

Secondary erythrocytosis is where an underlying condition causes more erythropoietin to be produced. This is a hormone produced by the kidneys that stimulates the bone marrow cells to produce red blood cells.

Health conditions that can cause secondary erythrocytosis include:

How erythrocytosis is diagnosed

Erythrocytosis can be diagnosed by carrying out a blood test to check:

A high concentration of red blood cells suggests you have erythrocytosis.

Erythrocytosis is sometimes only discovered during a routine blood test for another reason.

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

These may include:

Treatments for erythrocytosis

Treatment for erythrocytosis aims to prevent symptoms and complications (such as blood clots), and treat any underlying causes.

Venesection (removing blood)

Venesection is the simplest and quickest way of reducing the number of red cells in your blood. It may be recommended if you have polycythaemia vera, a history of blood clots, or symptoms suggesting your blood is too thick.

Venesection involves removing about 1 pint (half a litre) of blood at a time, in a similar way to the procedure used for blood donation.

How often this is needed will be different for each person. At first, you may need the treatment every week, but once your erythrocytosis is under control you should need it less often.

If you are going to have a venesection, your hospital will give you more information about what is involved.

Medicine to reduce red blood cells

In cases of polycythaemia vera, medicine may be prescribed to slow down the production of red blood cells.

Many different medicines 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 one for you. Examples include:

Medicine to prevent blood clots

If you have polycythaemia vera, daily low-dose aspirin tablets may be prescribed to help prevent blood clots and reduce the risk of serious complications.

You may also be offered treatment with low-dose aspirin if you have apparent or secondary erythrocytosis and another health problem affecting your blood vessels, such as coronary heart disease or cerebrovascular disease.

Treating and preventing other symptoms

Some people may also need treatment for any other symptoms or complications of erythrocytosis they have, or for any underlying cause of the condition.

For example, you may be given medicine to help relieve itching or manage COPD. Read more about:

Lifestyle changes that help

As well as improving some cases of apparent erythrocytosis, making healthy lifestyle changes can also reduce the risk of potentially serious blood clots for people with all types of erythrocytosis.

Having erythrocytosis means you're already at high risk of a blood clot, and being overweight or smoking only increases this risk.

You may find the following advice and information helpful:

Outlook for erythrocytosis

The outlook for erythrocytosis largely depends on the underlying cause.

Many cases are mild and may not lead to any further complications. However, some cases – particularly cases of polycythaemia vera – can be more serious and require long-term treatment.

If well controlled, erythrocytosis should not affect your life expectancy, and you should be able to live a normal life. However, people with polycythaemia vera can have a slightly lower life expectancy than normal due to the increased risk of problems, such as heart attacks and strokes.

Polycythaemia vera can also sometimes cause scarring of the bone marrow (myelofibrosis), which can eventually lead to you having too few blood cells. In some rare cases, it can develop into a type of cancer called acute myeloid leukaemia (AML).

Page last reviewed: 17 March 2023
Next review due: 17 March 2026