Updated 10 Aug 2017

In essential thrombocythaemia (ET) too many platelets are made in your bone marrow. This is the type of blood cell involved in blood clotting. ET usually develops very slowly and for the majority of people it doesn’t affect their normal lifespan.

What is essential thrombocythaemia (ET)?

All cells in your body contain a set of instructions that 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 ET, something goes wrong and causes a genetic fault to occur: you may hear your doctor talk about a fault, or mutation, in the JAK2, CALR or MPL genes. Genetic faults may happen because you’ve been exposed to hazardous chemicals, but more usually because of copying mistake when a cell was dividing. Around 60% of people with ET have the JAK2 genetic fault, 30% have the CALR fault and 5% have the MPL fault.

The JAK2 and MPL gene are 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. A growth factor is released when there are low levels of platelets in the blood.

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

The CALR gene was discovered in 2013. We don’t fully understand it yet, but we know it causes signals that lead to too many platelets being produced.

The presence of one of these genes in your blood might lead doctors diagnosing you with MPN. However, we‘re learning about these faulty genes, and their impact on treatment options and outlook, all the time. If you’d like to know more about those genes, your consultant will be happy to talk to you about them.

It’s important to note that these genetic faults happen during a person’s lifetime. As you’re not born with these faults, you can’t pass these 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 ET.


People who get ET are usually between 50 and 70 years old. The condition is rare in children, but can occur at any age.


Men and women are at equal risk of developing ET.

Symptoms and diagnosis

It’s likely that you won’t have any symptoms at all before or when you’re diagnosed. That’s why so many people with ET are diagnosed after a routine blood test. Older people and people with very high platelet counts get symptoms more often. Here are some symptoms you may experience:

  • persistent or repeated headaches,
  • disturbed vision (described by some patients as light shows or silent migraines),
  • dizziness or ringing in your ears,
  • bruising or bleeding easily (including heavy periods in women or nose bleeds),
  • erythromelalgia (pain and redness in some or all of your hands, feet, arms, legs, ears and face), and
  • your fingers or toes being blue, or feeling cold.


People with ET 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),
  • 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 higher in older patients who also have other medical conditions such as diabetes or heart problems, or in patients who have had clots in the past. However, the risk of thrombosis is reduced if your ET is treated appropriately.

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 ET

Most people are suspected of having ET after a routine blood test or by going to their GP with symptoms.

You’d then have a set of tests to confirm the diagnosis of ET.

Full blood count

In ET there’s an abnormally high level of platelets in the blood. A blood test known as a full blood count will detect if your platelet count is higher than normal.

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

Tests for genetic faults

DNA from one of your blood samples will be used to test for genetic faults to the JAK2, CALR and MPL genes. Around 60% of ET patients have a fault in the JAK2 gene. However, some people won’t have one of these faults, so a diagnosis can’t always be confirmed after these tests.

Bone marrow biopsy

Some people need tests on their bone marrow before their doctors can make a diagnosis. This helps to rule out any other bone marrow problems such as myelofibrosis (MF).

A small amount of bone marrow is taken using a needle from the hip bone. You don’t need to stay overnight in hospital for this; you can have it as an outpatient using local anaesthetic or mild sedation. It’s usually quite quick but will be uncomfortable while the sample’s being taken from the marrow; you can take painkillers if you need to. Your doctors will then look at the bone marrow sample under a microscope to assess it and look for any disease which might be in it.

Other causes of high platelet counts

If tests show that you don’t have any of the genetic faults linked with ET, your doctor will need to rule out other possible causes of a high platelet count before confirming a diagnosis. Other causes of a high platelet count can include:

  • unusual bleeding,
  • iron deficiency,
  • infections,
  • inflammatory diseases such as arthritis,
  • some types of cancer,
  • other blood cancers such as polycythaemia vera (PV), primary myelofibrosis (PMF) and chronic myeloid leukaemia (CML),
  • other types of cancer, and
  • if your spleen isn’t functioning normally.

Treatment and side effects

The treatment you receive for ET depends in part on your risk of developing complications. Patients are generally divided into low, intermediate or high-risk categories.

This is based on a combination of the following factors:

  • your age,
  • the symptoms you have,
  • your medical history (including risk factors for blood clots), and
  • your platelet count.

You should be monitored for cardiovascular risk factors such as diabetes, high cholesterol, high blood pressure and smoking, as these should be addressed as effectively as possible.


If you have a low risk of complications, you may be treated with aspirin.

Aspirin can help prevent clots because it affects the way platelets stick together. This doesn’t affect your platelet count, but helps to reduce the risk of blood clots. If you need to take painkillers for any other reason, ask your doctor which ones are safe.


You may have mild chemotherapy to treat your ET. Whether you have chemotherapy depends on a number of things, including:

  • your risk of thrombosis,
  • how well you’ll be able to cope with the side effects, and
  • your personal preference.


You may be prescribed a tablet called hydroxycarbamide (or hydroxyurea). This is a mild form of chemotherapy and works by reducing the number of platelets in your blood. Hydroxycarbamide is the most common chemotherapy drug used to treat ET. It does have some side effects: you may get diarrhoea, constipation or get more infections than usual.

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


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.


Interferon is an injection that slows down the production of platelets. It’s not thought to carry the same risk as hydrodroxycarbamide so is the preferred choice in younger patients. However, many patients find the short-term side effects unpleasant and not all patients 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.


Anagrelide is usually given only when other treatments have already been tried. It’s taken as a capsule but may increase the risk of developing myelofibrosis.


As ET is generally diagnosed in later life, for most people there’s a very good chance of living a normal lifespan if the condition is carefully monitored and treated.

It’s important to note that less than 5% of people with ET progress to a more aggressive disease such as 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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