What Is Thrombocytosis?

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Platelets are the cells in the body that are activated to help the blood clot and to heal blood vessel walls if they are damaged and bleeding. When there are too many platelets in the blood, this is referred to as thrombocytosis.

A normal platelet count is 150,000 to 450,000 platelets per microliter (mcL) of blood. A diagnosis of thrombocytosis is made when platelets are higher than 450,000 per mcL.

This article discusses the types, symptoms, causes, and diagnosis of thrombocytosis.

Vials of blood at a hospital

Yoshiyoshi Hirokawa / Getty Images

Types of Thrombocytosis

There are two general types of thrombocytosis:

  • Primary thrombocytosis happens as a result of the bone marrow making too many platelets.
  • Secondary thrombocytosis happens as a reactive process to something, such as infection, inflammation, or iron deficiency.

Thrombocytosis Symptoms

Very commonly, thrombocytosis does not show any symptoms and is found during routine blood work. However, it is possible that some symptoms may be present.

Symptoms may occur due to the platelets not functioning properly and leading to bleeding, while other symptoms may be related to platelets clumping together and causing blood clots.

These symptoms may include:

  • Bruising, usually with no known cause
  • Bleeding from places like the nose, gums, or seeing blood in the stool
  • An enlarged spleen
  • Burning in the hands or feet, usually worse when exposed to heat
  • Stroke-like symptoms: confusion, headache, vision changes, seizures, weakness to one side of the body
  • Symptoms of blood clots in the lungs: chest pain, shortness of breath, dry cough, palpitations
  • Symptoms of blood clots in the heart: chest pain, jaw pain, shortness of breath, palpitations, dizziness, arm pain
  • Symptoms of clots in blood vessels in the abdomen: severe abdominal pain, nausea and vomiting, bowel changes such as diarrhea, blood in the stool
  • Symptoms of a blood clot in an extremity: swelling to one extremity (usually a leg), warmth to the skin of the affected extremity, discomfort or cramping in the swollen extremity


In primary thrombocytosis, the high number of platelets being made is due to an abnormality in how the bone marrow is making them. This is also referred to as essential thrombocythemia, or ET. The bone marrow may not function normally due to a genetic mutation.

In ET, a mutation may exist in the JAK2 gene, which causes an enzyme to work too much at making platelets. A mutation in the CALR gene can also cause too many platelets to be made in the bone marrow.

Secondary thrombocytosis is also called reactive thrombocytosis, as the platelets rise in response to something outside of the bone marrow. Things that may lead to secondary thrombocytosis may include:

  • Infection
  • Inflammation or inflammatory diseases
  • Iron deficiency
  • Bleeding
  • Cancer
  • Recent surgery
  • Splenectomy (removal of the spleen)


Thrombocytosis is often found during routine blood testing, typically a complete blood count (CBC). This lab test would demonstrate a platelet count above 450,000 per mcL. Other testing that may be ordered to further evaluate the cause of thrombocytosis may include:

  • Iron panel and ferritin: These labs show how much iron is present.
  • C-reactive protein, erythrocyte sedimentation rate: These blood tests check for markers that can show if inflammation is present.
  • Blood tests for JAK2, CALR, or MPL gene
  • Bone marrow biopsy

A physical exam looking for bleeding, bruising, and evaluating spleen size may be done by a healthcare provider. These may provide some indication as to the cause of elevated platelets. 


Depending on the cause of thrombocytosis, there may be no long-term effects or life-threatening complications from having elevated platelet counts. 

However, if there is an underlying condition, such as cancer or another illness, or if complications from having too many platelets occur, these could impact long-term prognosis. 

Frequently Asked Questions

What can cause a high platelet count in children?

The most common cause of high platelets in children is an infection. It is much less likely to be caused by a primary bone marrow disorder.

What is essential thrombocytosis?

Essential thrombocytosis (ET) is a dysfunction in the bone marrow that causes too many platelets to be made. In ET, a mutation may exist in the JAK2 gene, which causes an enzyme to work too much at making platelets. A mutation in the CALR gene can also cause too many platelets to be made in the bone marrow.

What is reactive thrombocytosis?

Reactive thrombocytosis is an elevation in platelet counts for a reason outside of the bone marrow. The platelets increase in reaction to something such as iron deficiency, infection, inflammation, cancer, or bleeding. 

A Word From Verywell

Being told that you have high platelets may make you feel anxious or nervous. Remember that the most common cause of high platelets is due to a reactive process. If you continue to be concerned or have additional questions, be sure to reach out to your healthcare team for answers. 

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Johns Hopkins Medicine. What are platelets and why are they important?

  2. National Heart, Lung, and Blood Institute. Thrombocythemia and thrombocytosis.

  3. Cleveland Clinic. Thrombocytosis.

  4. National Organization for Rare Disorders. Essential thrombocythemia.

  5. Watson J, Jones HE, Banks J, Whiting P, Salisbury C, Hamilton W. Use of multiple inflammatory marker tests in primary care: using Clinical Practice Research Datalink to evaluate accuracyBr J Gen Pract. 2019;69(684):e462-e469. doi:10.3399/bjgp19X704309

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By Julie Scott, MSN, ANP-BC, AOCNP
Julie is an Adult Nurse Practitioner with oncology certification and a healthcare freelance writer with an interest in educating patients and the healthcare community.