What Causes a High Platelet Count?

There are many reasons why your platelet counts could be high, most of which are short-lived and nothing to worry about. However, there are occasions when a higher platelet count—referred to as thrombocytosis or thrombocythemia—could be a sign of a serious medical condition, such as an infection, an inflammatory disease, or even cancer.

High platelet counts (defined as having more than 400,000 platelets per microliter of blood) are broadly categorized as being either:

This article explains the difference between reactive thrombocytosis and essential thrombocythemia as well as the underlying conditions that trigger high platelet counts.

An illustration with causes of a high platelet count

Illustration by Michela Buttignol for Verywell Health

Reactive Thrombocytosis

Sick woman taking her temperature
Sam Edwards / Getty Images

Reactive thrombocytosis (RT), also known as secondary thrombocytosis, is the more common cause of a high platelet count. With RT, platelet counts will rise as a result of an underlying event or disease.

The causes of RT are described in seven broad categories:


Infections are the most common cause of a high platelet count in both adults and children.

Soft tissue, lung, gastrointestinal, urinary, and genital infections are the most common infectious causes of RT. The majority are caused by the following bacterial species or strains:

Platelet counts generally return to normal after the infection resolves, but this may take several weeks.

Chronic Inflammatory Diseases

Inflammation is the body's natural response to an infection or injury. When confronted with inflammation, the immune system will increase the production of platelets to aid with the repair of damaged tissues.

Numerous chronic (persistent) inflammatory diseases are associated with RT, including:

The increase in platelets usually occurs during acute exacerbations (flare-ups) or when the disease is being undertreated.

Allergic Conditions

Allergy and atopic (allergy-related) diseases can also cause RT. Allergies are caused by an abnormal immune response that can spur systemic (whole-body) inflammation and an associative increase in platelets.

Allergic and atopic conditions associated with high platelet counts include:

Thrombocytosis in these instances tends to be transient and will resolve once the allergy is brought under control.


Iron-deficiency anemia occurs when the lack of iron in the blood causes a decrease in the number, size, and function of red blood cells. 

With iron-deficiency anemia, there will be a characteristic drop in red blood cells and hemoglobin (the protein in red blood cells that transport oxygen to cells). For reasons that are not entirely clear, platelet counts will commonly rise when this occurs.

Blood loss, a lack of dietary iron, pregnancy, and intestinal diseases that affect the absorption of iron are the most common causes of iron-deficiency anemia.

Another type of anemia associated with RT is hemolytic anemia in which red blood cells are destroyed as a result of conditions like cancer, hepatitis, sickle cell disease, or certain autoimmune diseases. Again, scientists are not entirely sure why platelets increase as the red blood count decreases.


Asplenia is the lack of a spleen. The spleen works as a blood filter, removing old red blood cells and holding onto a reserve of other blood cells in the event of an emergency, such as a hemorrhage.

All told, roughly one-third of the body's total platelets are stored in the spleen at any one time.

If your spleen stops functioning properly—due to a condition like sickle cell disease or an injury—or is surgically removed (splenectomy), your platelet count can increase to compensate for the loss of its reserve.


Certain medications are known to affect the normal balance of blood cells and cause an increase in platelets. Some do so by disrupting the normal function of the bone marrow, while others cause side effects like bleeding that lead to anemia.

Drugs associated with an increased risk of RT include:


RT is common with many forms of cancer. Particularly with solid tumor cancers, the progression of the disease will often trigger an associative increase in platelets, referred to as paraneoplastic thrombocytosis. These increases can be extreme when a tumor has metastasized (spread to distant parts of the body).

Paraneoplastic thrombocytosis can occur with all types of cancer but is especially common with breast, colon, ovarian, cervical, kidney, testicular, and lung cancer.

Blood cancers like lymphoma and chronic myeloid leukemia (CML) are also characterized by abnormally high platelet counts.

Essential Thrombocythemia

Human Red blood cells
Micro Discovery / Getty Images

Essential thrombocythemia (ET), also known as primary thrombocythemia, is a less common cause of high platelet counts. ET is a disease in and of itself and one that is thought to be influenced by a person's genetics.

Essential thrombocythemia belongs to a group of rare blood cancers called myeloproliferative neoplasms, which cause the bone marrow to produce too many blood cells. With essential thrombocythemia, too many platelets are produced.

ET is rare, affecting only around three of every 100,000 people annually. Studies suggest that around half of all people with ET have a specific mutation of a gene called JAK2.

ET is a slowly progressive disease with long periods with no symptoms. When symptoms do develop, they tend to manifest with bleeding or blood clots (in some cases causing deep vein thrombosis or pulmonary embolism). In rare instances, ET can progress to acute myeloid leukemia (AML).

ET is more common in older adults, especially females, and is rarely seen in children.

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

  2. Rose SR, Petersen NJ, Gardner TJ, Hamill RJ, Trautner BW. Etiology of thrombocytosis in a general medicine population: analysis of 801 cases with emphasis on infectious causesJ Clin Med Res. 2012;4(6):415-423. doi:10.4021/jocmr1125w

  3. World Health Organization. Anaemia.

  4. National Heart, Lung, and Blood Institute. Thrombocytopenia.

  5. Oida T, Kano H, Mimatsu K, et al. Thrombocytosis following splenectomy: with or without additional organ resection. Hepatogastroenterology. 2012;59(116):1033-5. doi:10.5754/hge10032

  6. Brière JB. Essential thrombocythemia. Orphanet J Rare Dis. 2007;2:3.  doi:10.1186/1750-1172-2-3

By Amber Yates, MD
Amber Yates, MD, is a board-certified pediatric hematologist and a practicing physician at Baylor College of Medicine.