Essential Thrombocythemia (ET): Understanding Your Pathology Report

Section Editor: David Li MD
June 25, 2026

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 bones where blood cells are made. In essential thrombocythemia, the bone marrow produces too many platelets, the small cell fragments in the blood that help form clots to stop bleeding. When platelet counts remain 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 return to normal.

This article will help you understand the findings in your pathology report for essential thrombocythemia, what each term means, and why it matters for your care.

What are the symptoms of essential thrombocythemia?

Many people with essential thrombocythemia (ET) have no symptoms and only learn about the condition when a blood test shows a high platelet count. Others develop symptoms caused by too many platelets or by changes in blood flow. These may include headaches, dizziness, redness or burning pain in the hands and feet (a symptom called erythromelalgia), trouble concentrating, and fatigue.

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 a clot in the deep veins of the leg, a clot in the lungs, a heart attack, or a stroke. Clots may also develop in less common locations, such as the veins of the abdomen, and 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 very high. This occurs because extremely high platelet counts can interfere with normal clotting, leading to a temporary condition resembling von Willebrand disease (a bleeding disorder). Bleeding may involve the nose, mouth, or gastrointestinal tract.

How common is essential thrombocythemia?

Essential thrombocythemia (ET) is uncommon, with roughly 1 to 2 people per 100,000 diagnosed each year. It is most often found in older adults but can occur at any age, and among younger patients it is somewhat more common in women.

What causes essential thrombocythemia?

The cause of essential thrombocythemia (ET) is not known. Most people develop a mutation (change) in a gene that regulates blood cell growth. These changes occur during life and are not inherited. The mutation causes the bone marrow to make more platelets than the body needs. The specific genes involved are described in the next section.

What genetic changes are involved in essential thrombocythemia?

Most cases of essential thrombocythemia (ET) are driven by a mutation in one of three genes, and these mutations are usually found one at a time rather than together:

JAK2 — The most common, found in about 50 to 60% of cases (most often the JAK2 V617F mutation).
CALR — Found in about 25 to 35% of cases, usually described as type 1 or type 2.
MPL — Found in about 5 to 10% of cases.

Finding one of these mutations supports the diagnosis because all three genes help control platelet production, and a mutation causes the gene to send constant signals to make platelets. About 10 to 15% of people have none of these three mutations, a situation sometimes called triple-negative; these people can still be diagnosed with essential thrombocythemia if the blood and bone marrow findings match and other causes have been ruled out. Testing is usually performed on blood or bone marrow, most often using next-generation sequencing (NGS), and the report may include the variant allele frequency, the percentage of cells carrying the mutation. A dedicated article explains JAK2 testing in more detail.

How is the diagnosis made?

The diagnosis of essential thrombocythemia (ET) is made by reviewing blood tests, examining the bone marrow, and performing genetic testing, while ruling out other conditions that can cause a high platelet count. Blood tests typically show a high platelet count, and a blood smear (a thin layer of blood examined under a microscope) often shows increased platelets that vary in size, some unusually large. The red and white blood cells are usually normal, although iron deficiency from repeated bleeding can make red blood cells appear small and pale.

A bone marrow biopsy is often performed to help confirm the diagnosis. The bone marrow usually has a normal number of cells for the person’s age, and the key feature is an increased number of megakaryocytes, the large cells that produce platelets. In essential thrombocythemia, these megakaryocytes are typically very large and mature-looking, with deeply folded, “staghorn-shaped” nuclei, and they may cluster loosely. Scarring (fibrosis) is usually absent or mild at diagnosis, the other blood-forming cells look normal, and immature cells called blasts are not increased. Special stains such as CD61 or CD42b may be used to highlight the megakaryocytes. Genetic testing for the JAK2, CALR, or MPL mutations described above supports the diagnosis.

Before confirming essential thrombocythemia, doctors rule out other causes of a high platelet count. Reactive thrombocytosis is a temporary rise in platelets in response to another condition, such as infection, iron deficiency, inflammation, or recent surgery. Other myeloproliferative neoplasms, including polycythemia vera and primary myelofibrosis, can also show increased megakaryocytes, and chronic myeloid leukemia can cause a high platelet count as well. Testing for the BCR::ABL1 fusion gene (the Philadelphia chromosome) is done to rule out chronic myeloid leukemia, because that condition is treated differently.

What happens after a diagnosis of essential thrombocythemia?

After essential thrombocythemia (ET) is diagnosed, the main goal of treatment is to lower the risk of blood clots while avoiding unnecessary bleeding. The care team first estimates the risk of clotting, which is considered higher in people aged 60 or older, in anyone who has already had a blood clot, in those with a JAK2 mutation, and in those with cardiovascular risk factors such as high blood pressure, diabetes, or smoking. The pathology and blood findings help guide several decisions:

Low-dose aspirin — Often used to lower the risk of clots and to ease symptoms such as headaches and burning pain in the hands and feet. It is sometimes withheld when the platelet count is extremely high, because at very high counts the blood can have a greater tendency to bleed.
Cytoreductive therapy — For higher-risk disease, medicines that lower platelet production may be added. First-line options include hydroxyurea and interferon (interferon is often preferred for younger patients and during pregnancy), with anagrelide as another option.
Managing other risks — Addressing cardiovascular risk factors such as blood pressure, cholesterol, and smoking, which also affect the risk of clotting.

Most people with essential thrombocythemia continue regular follow-up with blood tests and clinical assessments to monitor their counts and symptoms and to watch for any signs of progression. Newer treatments are being studied in clinical trials, which may be an option to discuss. Decisions about whether and how to treat are made by the care team together with the patient, based on the specific findings in the report.

What is the prognosis for essential thrombocythemia?

Essential thrombocythemia (ET) 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 and, less commonly, bleeding, both of which can usually be reduced with appropriate treatment and careful monitoring.

About 10 to 15% of people develop bone marrow scarring (post-essential thrombocythemia myelofibrosis) after many years, and a smaller number progress to acute myeloid leukemia. The risk of these complications depends on factors such as age, anemia, specific genetic changes, and white blood cell count. Doctors use risk scores to help decide who may benefit from treatment, but these scores estimate the risk of clotting and bleeding rather than survival. Your prognosis depends on your own combination of these factors, which your care team can explain in the context of your specific report.

Questions to ask your doctor

Did my genetic tests show a mutation such as JAK2, CALR, or MPL?
Was I tested for the BCR::ABL1 fusion gene to rule out chronic myeloid leukemia?
What did my bone marrow biopsy show, including whether there was any scarring?
Am I considered low risk or high risk for blood clots?
Should I be taking low-dose aspirin?
Is my platelet count high enough to increase my risk of bleeding?
Do I need a medication to lower my platelet count, and if so, which one?
How often should I have follow-up blood tests?
What symptoms should I watch for?
What signs might suggest progression to myelofibrosis or leukemia?
Are there clinical trials that I should consider?