Polycythaemia, or erythrocytosis, 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 polycythaemia are caused by this sluggish flow of blood. 

Signs and symptoms

Mild cases of polycythaemia may not cause any problems, but some people with polycythaemia can experience:


blurred vision

red skin, particularly in the face, hands and feet 


high blood pressure


discomfort in the tummy (abdomen)

periods of confusion

bleeding problems – such as nosebleeds and bruising

gout – which can cause joint pain, stiffness and swelling

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 

You should make an appointment to see your GP if you have persistent symptoms of polycythaemia.

When to seek immediate medical advice

The slow blood flow associated with polycythaemia can also causeblood clots. These can be serious because they can put you at risk of life-threatening problems such as heart attacks, pulmonary embolisms(a blockage in the blood vessel that carries blood from the heart to the lungs) and strokes.

In some cases, a blood clot – known as deep vein thrombosis (DVT) – may form in your leg, before moving elsewhere in your body. Signs of DVT or a pulmonary embolism can include:

pain, swelling, redness and tenderness in one of your legs

a heavy ache in the affected area

warm skin in the area of the clot


chest or upper back pain

coughing up blood

feeling lightheaded or dizzy


If you experience any of the above symptoms, seek medical help immediately. You should also seek emergency medical help if you think you or someone you are with is having a heart attack or stroke.

What causes polycythaemia?

Polycythaemia can be divided into several different types, depending on the underlying cause of the condition. In some cases, an underlying cause cannot be identified.

Apparent polycythaemia

"Apparent polycythaemia" 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.

The condition is often caused by being overweight, smoking, drinking excessive amounts of alcohol or taking certain medications – such as diuretics. A similar condition that is sometimes called "relative polycythaemia" can also occur as a result of dehydration.

Apparent polycythaemia may improve if the underlying cause is identified and managed. For example, stopping smoking or reducing your alcohol intake may help.

Absolute polycythaemia

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

primary polycythaemia – where there is a problem in the cells produced by the bone marrow that become red blood cells; the most common type is known as polycythaemia vera (PV)

secondary polycythaemia – where too many red blood cells are produced as the result of an underlying condition

Both PV and secondary polycythaemia are described in more detail below.

Polycythaemia vera (PV)

PV is a rare condition usually caused by a fault in a gene called the JAK2 gene, which causes the bone marrow cells to produce too many red blood cells.

The affected bone marrow cells can also develop into other cells found in the blood, which means people with PV may also have abnormally high numbers of both platelets (thrombocytosis) and white bloods cells (leukocytosis).

Although caused by a genetic fault, PV is not usually inherited. Most cases develop later in life, with 60 the average age of diagnosis.

Secondary polycythaemia

Secondary polycythaemia 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.

Conditions that can cause secondary polycythaemia include:

chronic obstructive pulmonary disease (COPD) and sleep apnoea – these can cause an increase in erythropoietin, due to not enough oxygen reaching the body's tissues

a problem with the kidneys – such as a kidney tumour or narrowing of the arteries supplying blood to the kidneys

How polycythaemia is diagnosed

Polycythaemia can be diagnosed by carrying out a blood test to check the number of red blood cells in your blood (red blood cell count) and the amount of space the red blood cells take up in the blood (haematocrit level). A high concentration of red blood cells in your blood suggests you have polycythaemia.

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

Your GP may refer you to a haematologist (specialist in conditions affecting the blood) for further tests to confirm the diagnosis and to determine the underlying cause, such as a blood test to look for the faulty JAK2 gene and an ultrasound scan of your abdomen to look for problems in your kidneys.

Treating polycythaemia

Treatment for polycythaemia aims to prevent symptoms and complications (such as blood clots), and treat any underlying causes. Some of the main treatments used are described below.


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

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

How often this is required will be different for each person. At first you may need the treatment every week, but once your condition is under control you may only need it every 6-12 weeks or less.

For more information, read an leaflet on having a venesection (PDF, 335kb). 

Medication to reduce the production of red blood cells

In cases of PV, medication may be prescribed to slow down the production of red blood cells.

Many different medications are available that can help do this, 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:

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

Medication to prevent blood clots

Daily low-dose aspirin tablets may be prescribed to help prevent blood clots and reduce the risk of serious complications, if you have PV.

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

Treating and preventing other conditions

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

For example, you may be given medication to help relieve itching, manage COPD or treat gout. 

Lifestyle changes

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

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:

losing weight

healthy weight calculator

preventing cardiovascular disease

managing high blood pressure

stopping smoking


The outlook for polycythaemia largely depends on the underlying cause.

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

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

PV 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, the condition can develop into a type of cancer called acute myeloid leukaemia (AML).

If you have polycythaemia, it is important to take any medication you are prescribed and keep an eye out for signs of possible blood clots to help reduce your risk of serious complications.




Polycythaemia means having a high concentration of red blood cells in your blood 


Blood clot warning signs

If you have polycythaemia, it's important to look out for signs of blood clots, such as:

pain, swelling, redness and tenderness in one of your legs

a heavy ache in the affected area

warm skin in the area of the clot


chest or upper back pain

coughing up blood

feeling lightheaded or dizzy


If you experience these symptoms, get medical help immediately.