by Jason Wasserman MD PhD FRCPC and Kamran M. Mirza MBBS PhD
December 12, 2025
Essential thrombocythemia (ET) is a type of blood cancer called a myeloproliferative neoplasm. This group of conditions starts in the bone marrow, the soft, spongy tissue inside your bones where blood cells are made. In essential thrombocythemia, the bone marrow produces too many platelets. Platelets are small cell fragments in the blood that help form clots to stop bleeding. When platelet counts remain extremely high for a long time, they can cause problems such as abnormal clotting or, less commonly, bleeding.
Essential thrombocythemia is most often found in adults, and many people feel well at the time of diagnosis. It is usually discovered after routine blood work shows a high platelet count that does not go back to normal.
What are the symptoms of essential thrombocythemia?
Some people have no symptoms and only learn about the condition when a blood test shows a high platelet count. Others may develop symptoms caused by too many platelets or by changes in blood flow. These symptoms may include headaches, dizziness, redness or burning pain in the hands and feet (erythromelalgia), or trouble concentrating. Fatigue is also common.
Essential thrombocythemia can increase the risk of abnormal blood clots. These clots may form in veins or arteries and can cause serious events such as deep vein thrombosis, pulmonary embolism, heart attack, or stroke. Clots may also develop in less common areas such as the portal vein or splenic vein. In some people, these unusual clots are the first sign of an underlying myeloproliferative neoplasm.
Bleeding can also occur, especially when the platelet count becomes extremely high. This can happen because high platelet counts sometimes interfere with normal clotting, leading to a temporary condition similar to von Willebrand disease. Bleeding may involve the nose, mouth, or gastrointestinal tract.
Although many people remain stable for years, a small number may develop a more advanced disease called post-essential thrombocythemia myelofibrosis. Very rarely, essential thrombocythemia progresses to acute myeloid leukemia. These changes usually develop slowly over many years.
What causes essential thrombocythemia?
The cause is not known. Most people with essential thrombocythemia develop a mutation (change) in a gene that regulates blood cell growth. These gene changes occur during life and are not inherited.
The most common genetic change involves the JAK2 gene. Mutations in CALR or MPL are also found. These mutations make the bone marrow produce more platelets than the body needs. Some people do not have any of these mutations, and in those cases, the cause remains unknown.
How is the diagnosis made?
Doctors diagnose essential thrombocythemia by reviewing blood tests, performing a bone marrow biopsy, and conducting genetic testing. They must also rule out other conditions that can cause a high platelet count.
Blood tests
Blood tests usually show a high platelet count. A blood smear, a thin layer of blood examined under a microscope, often shows increased platelet numbers and may show platelets of varying sizes. Some may appear unusually large. The red and white blood cells are usually normal, although iron deficiency (sometimes caused by repeated bleeding) may lead to small, pale red blood cells.
Bone marrow biopsy
A bone marrow biopsy is often performed to help confirm the diagnosis. Under the microscope, the bone marrow usually has a normal number of cells for the person’s age. The key feature is an increased number of megakaryocytes, the large bone marrow cells that produce platelets. In essential thrombocythemia, these megakaryocytes are often very large with a mature appearance and deeply folded or “staghorn-shaped” nuclei.
Scarring in the bone marrow (called fibrosis) is usually absent or mild at diagnosis. The other types of blood-forming cells typically look normal, and the number of young immature cells (called blasts) is not increased.
Special stains such as CD61 or CD42b may be used to highlight megakaryocytes.
Genetic tests
Genetic testing looks for mutations in genes commonly associated with essential thrombocythemia. These tests are usually performed on a blood sample or on bone marrow taken during a biopsy. Most laboratories use next-generation sequencing (NGS) to detect specific gene changes.
The report may list results in several different ways. A mutation may be described by its full scientific name, such as JAK2 p.V617F, CALR type 1, CALR type 2, or MPL p.W515L. Some labs also provide the variant allele frequency, which is the percentage of tested cells that carry the mutation.
Finding a mutation in JAK2, CALR, or MPL supports the diagnosis of essential thrombocythemia because these genes help control platelet production. A mutation means that the gene sends constant signals to make platelets, even when the body does not need them. People without a detectable mutation can still be diagnosed with essential thrombocythemia if blood and bone marrow findings match the disease and other causes have been ruled out.
Other conditions to rule out
Doctors must also rule out conditions that can mimic essential thrombocythemia, such as reactive thrombocytosis. Reactive thrombocytosis occurs when the body produces extra platelets in response to another condition, such as infection, iron deficiency, inflammation, or recent surgery. Other bone marrow diseases, including polycythemia vera and primary myelofibrosis, may also show increased megakaryocytes. Distinguishing among these conditions is important because they behave differently over time.
What is the prognosis for a person with essential thrombocythemia?
Essential thrombocythemia is generally considered an indolent, or slow-growing, disease. Many people live for decades after diagnosis and may have a near-normal life expectancy. The most important risks are abnormal blood clots or, less commonly, bleeding. These events can usually be reduced with appropriate treatment and careful monitoring.
About 10 to 15 percent of people develop bone marrow scarring (post-essential thrombocythemia myelofibrosis) after many years. A smaller number progress to acute myeloid leukemia. The risk of these complications depends on several factors, including age, anemia, specific genetic changes, and white blood cell count.
Doctors use risk scores to help decide who may benefit from treatment to lower the platelet count or reduce the risk of clotting. However, these scores are based on risk of clotting and bleeding, not survival. Most people with essential thrombocythemia continue regular follow-up with their doctor to monitor their blood counts and symptoms.
Questions to ask your doctor
- Did my genetic tests show a mutation such as JAK2, CALR, or MPL?
- What is my risk of blood clots or bleeding?
- Do I need treatment to lower my platelet count?
- How often should I have follow-up blood tests?
- What symptoms should I watch for?
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