Essential thrombocythaemia (ET)

Essential thrombocythaemia (ET) is one of the myeloproliferative neoplasms (MPNs) where patients have too many platelets in their blood.

Essential thrombocythaemia (ET) is one of a group of diseases called the myeloproliferative neoplasms (MPNs). MPNs affect the way blood cells are produced in the body. Patients with ET have too many platelets in their blood. Platelets are small fragments of cells which are needed to form a blood clot. The high number of platelets causes people with ET to have a higher risk of blood clotting. People with ET can also have bleeding problems because their platelets do not always work well. People generally develop ET as adults and most people who develop ET are diagnosed at age 60 or older.

What causes ET?

There are many conditions, other than ET which can cause a raised platelet count, such as arthritis. Before diagnosing ET, doctors will rule out these other causes.

There are no definite causes of ET, but there are some known risk factors which increase the likelihood  – of a person developing ET. These include:

  • age – as with most blood cancers the risk increases with age.
  • gene mutations – it has been shown that about half of all patients with ET have a mutation (or change) in a gene called JAK2. Most patients who do not have the JAK2 change, have changes in one of two genes called CALR or MPL. Researchers don’t currently understand what causes these mutations to occur but the mutations are not passed on from parent to child.

It is very unusual, although not unknown, for more than one person in a family to develop ET.


Signs and symptoms of ET

Having too many platelets in the blood may not cause any symptoms, and about half of all people are diagnosed with ET during a routine blood test, when they have no symptoms.

Possible symptoms of ET include:

  • headaches
  • dizziness
  • redness, swelling or pain in the hands or feet due to reduced blood flow
  • tiredness
  • skin itching

Sometimes symptoms develop due to complications of ET, such as a blood clot caused by too many platelets in the blood. Less commonly, people may have bleeding or bruising because the platelets in their blood don’t work properly.

These symptoms aren’t always caused by ET and symptoms can vary from person-to-person. If you have any of them or are concerned at all, you should contact your GP.

Diagnosis of ET

A diagnosis of ET may be suspected following a routine blood test for something else, but is confirmed by a haematology specialist who will run a series of diagnostic tests to determine whether you have ET or another condition. It’s possible for conditions other than ET to cause a high platelet count such as arthritis and other blood conditions, so your haematologist will need to rule out any other possible underlying causes.

ET is often suspected if a routine blood test shows that a patient has a high platelet count, also called thrombocytosis.

To make a diagnosis of ET, the following criteria must be present:

  • A platelet count of 450×109/L or higher
  • A bone marrow biopsy showing an increase in the numbers of enlarged, mature megakaryocytes (cells in the bone marrow that produce platelets)
  • Not meeting the diagnosis for another MPN, such as PV or MF
  • The presence of a JAK2, CALR or MPL mutation


If none of the mutations are present, other causes of primary or secondary thrombocytosis as well as diagnoses of PV and MF must be excluded. Primary refers to a condition that is the root cause of the illness, whereas secondary refers to a condition that has developed as a result of another one.


ET is diagnosed using laboratory tests including:

  • Blood tests – Blood tests can identify an increase in blood cells and exclude other causes of a high cell count.
  • Gene mutation analysis – This is to identify any gene mutations you may have, particularly JAK2, CALR or MPL.
  • Bone marrow investigations – You may have either a bone marrow aspiration or both an aspiration and a bone marrow biopsy. This will be done to look for classic signs of ET. During a bone marrow aspiration, the doctor or nurse takes some bone marrow cells up into a syringe. A bone marrow biopsy is when they remove a one to two-centimetre core of bone marrow in one piece using a trephine (a surgical instrument with a cylindrical blade). The sample is then sent to the laboratory for testing.

Tests for ET

If your haematologist suspects you have ET, they may suggest some of the following tests:

  • Full blood count (blood test)

This test counts the number of red blood cells, white blood cells and platelets. In ET there will be a high platelet count.

  • JAK2, CALR and MPL mutation test

Your haematologist can test your blood to see if you have a gene change (or mutation) in genes called JAK2, CALR and MPL. Most people with ET have one of these mutations. About half of all people with ET have a mutation in the JAK2 gene. This change in the gene happens during the person’s lifetime. It isn’t something you’re born with and you can’t pass it on to your children. People who do not have a mutation in JAK2 might be checked for a mutation in another gene called calreticulin (CALR). As with JAK2 mutations, change in the gene happens during the person’s lifetime. It isn’t something you’re born with and you can’t pass it on to your children.

  • Abdominal ultrasound to check for abnormalities with your spleen which may have become enlarged.
  • Bone marrow biopsy

Your doctor may want to take a sample of marrow (biopsy) to check for fibrosis (scarring) in the bone marrow. The sample is usually taken from the back of your hip bone (pelvis). You’ll be given an injection of local anaesthetic to numb the area. The doctor will then pass a needle through the skin into the bone, and draw a small sample of liquid marrow (called a bone marrow aspirate) into a syringe. After this, the doctor will take a small core of marrow from the bone (a trephine biopsy). Both samples will be looked at later under a microscope.

Complications of ET

Blood clots

Symptoms can occur because a blood clot (thrombosis) has formed in a vein in the body. Clotting problems are more common in people over 60, in people who have had a blood clot before, and in people who have conditions such as high blood pressure or diabetes. If you receive a diagnosis of ET, your haematologist will assess the risk of developing a blood clot, which is the most common, serious complication of ET. Your haematologist will consider your risk level when making recommendations about your treatment plan.

Symptoms of a blood clot depend on where it is in the body. These are some common sites where a clot may form:

  • veins of the leg – causing pain, swelling, heat and redness, usually in the calf. This is known as deep vein thrombosis or DVT.
  • the brain – this may cause mild symptoms such as headaches, blurred vision, or dizziness. Blood clots in the brain may also cause a stroke or mini stroke (also called a transient ischaemic attack or a TIA).
  • the heart – a blood clot in the heart may cause chest pain and, in severe cases, can result in a heart attack.
  • the lungs – a blood clot in a lung may cause breathlessness and chest pain. This is known as a pulmonary embolism.

Treatment of ET

The goal of ET treatment is reducing the risk of potential complications, such as blood clots, while keeping side effects to a minimum. The first step in any treatment plan is to take good care of yourself. Maintaining a healthy body weight and controlling your blood pressure and cholesterol can go a long way toward protecting your long-term health.

In the early stages of ET you may not have any symptoms and you may be considered low risk. If this is the case, you may not be treated straightaway but will be monitored by your medical team for any sign of changes. This is referred to as ‘watch and wait’.

ET patients are risk-classified according to the expected chance of clotting or bleeding, so the first goal of treatment is to reduce the likelihood of these events. Haematologists can use a number of different drugs to treat ET, which can include:

Medications to reduce your risk of clotting:

  • Aspirin – if you have a low risk of complications, you may be treated with aspirin which affects the way platelets ‘stick together’, and therefore prevents clots.
  • Warfarin – a blood thinner (anticoagulant) which can be given only if you have previously had a clot.

Treatment to reduce your platelet count:

  • Chemotherapy

This is the use of anti-cancer drugs to destroy cancer cells. Chemotherapy can also be used to treat non-cancer conditions and for ET it can be given to reduce the number of platelets. Hydroxycarbamide is the most commonly used chemotherapy drug to treat ET. Side effects can include lowered resistance to infection, a reduced number of red blood cells (anaemia) , diarrhoea or constipation. The chemotherapy drug busulfan may sometimes be used and can cause similar side effects to hydroxycarbamide.

  • Interferon alpha

Interferon slows down the production of blood cells, including platelets. It is used most often to treat ET in younger people (under 40), although it can be used at any age. Interferon is given as an injection under the skin. Possible side effects include flu-like symptoms, headaches, dizziness, mood swings and tiredness.

  • Anagrelide

Anagrelide is usually given when other treatments have already been tried. Side effects can include headaches and palpitations (a fast heartbeat).

  • Radioactive phosphorus (P32)

Radioactive phosphorus may be used to treat ET when other treatments have not worked or are unsuitable. It is given as an injection into a vein. It irradiates the bone marrow and can lower the number of platelets being made. The effects of a single injection can last from months to years.

We understand going through a blood cancer journey can be difficult. It may help to talk to a close friend or relative about how you are feeling. Here are a list of questions that may be useful to ask your medical team.

Questions to ask your medical team about ET

  • How can you tell if I have essential thrombocythaemia (ET)?
  • What does that mean for me? What tests will I need to have?
  • What will the tests show?
  • How long will the results take?
  • What sort of treatment will I need?
  • How long will my treatment last?
  • What happens if the treatment doesn’t work?
  • What will the side effects be?
  • Can these be managed?
  • Will I need to be off work/college?
  • Where can I get help dealing with my feelings?

Further downloads

We have free patient information available for ET patients.

You can download the booklets on our information pages here.

Alternatively, you can have the information delivered free of charge by requesting it through our resources page. 

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