What Causes a High Platelet Count?

Platelets are cells in your blood that clump together and form clots to stop bleeding. A platelet count that is above the normal, healthy range—that is, more 450,000 cells per microliter—is considered elevated. A high platelet count is known as thrombocytosis.

There are many reasons why your platelet counts could be high, most of which are acute and nothing to worry about. However, a high platelet count can be serious if it's a sign of disease or other health condition, such as cancer or inflammatory bowel disease (IBD). When high platelet counts are the result of another condition or disease, it's known as secondary thrombocythemia.

This article lists eight possible conditions that can cause an elevated platelet count, along with how each condition may be treated.


Sick woman taking her temperature
Sam Edwards / Getty Images

Infections are the most common cause of a high platelet count in both children and adults. This elevation can be extreme, with platelet counts greater than 1 million cells per microliter.

In one study, the most frequent types of bacteria that cause high platelets are methicillin-resistant Staphylococcus aureus (MRSA), Clostridium difficile, Enterococcus species, Pseudomonas species, and Escherichia coli.

Most people don't have symptoms if they're experiencing high platelet counts during an infection. But a small group of patients with other risk factors may develop blood clots.

Platelet counts generally return to normal after the infection resolves, but this may take several weeks. In some patients, thrombocytosis may be an effect of having thrombocytopenia (low platelets) during the initial infection.

Myleoproliferative Disorders

Human Red blood cells
Micro Discovery / Getty Images

Chronic myeloproliferative disorders (conditions where the bone marrow makes too many blood cells) can cause high platelet counts.

These include polycythemia vera, essential thrombocythemia (ET, also known as primary thrombocythemia), and primary myelofibrosis. In ET, for example, the bone marrow makes too many megakaryocytes, the cells that make platelets, resulting in thrombocytosis.

With these conditions, the high platelet count causes the blood to be thick and flow more slowly, which may result in blood clots. Treatment is aimed at reducing the platelet count to reduce this risk.

Myeloproliferative disorders are more common in adults and are rarely seen in children.

Iron Deficiency Anemia

Anaemia illustration


Although low hemoglobin and small red blood cell counts are the typical laboratory values associated with iron deficiency anemia, it is not uncommon to also have an elevated platelet count.

Young children and women who are pregnant are more likely to develop anemia, but the condition can impact people of all ages. Diet and some underlying conditions (such as ulcers, which can cause blood loss) can increase your risk of developing anemia.

Experts do not know what exactly causes this type of thrombocytosis. In general, it is well tolerated and resolves with appropriate iron supplementation treatment.​

Not Having a Spleen

Human pancreas, gallbladder, and spleen, illustration

About one-third of your body's platelets are stored in the spleen—the body's blood filter.

If your spleen stops functioning properly—due to a condition like sickle cell disease or an injury—or if you have it removed surgically (splenectomy), your platelet count could go up. This thrombocytosis is usually mild to moderate and well-tolerated.

Immediately after spleen removal, platelet counts may be severely high and could trigger the formation of a blood clot. This post-splenectomy complication occurs in about 5 percent of patients. In general, the risk is greatest in the month after splenectomy.

Inflammatory Conditions

Illustration of an intestine with Crohn's Disease

JUAN GARTNER / Science Photo Library / Getty Images

Conditions with inflammation, like rheumatologic disorders, inflammatory bowel disease, vasculitides, and some autoimmune conditions (like vasculitides), can also cause high platelet counts.

In most cases, chronic inflammation is seen more often in adults than in children.

The elevated platelet count occurs in response to cytokines (small "messenger" proteins released from cells that signal other cells to do something). In particular, the cytokines interleukin-6 and thrombopoietin stimulate platelet production.

Platelets are considered alongside inflammatory markers like C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) when working toward a diagnosis.

Mixed Cryoglobulinemia

Hepatitis C virus, illustration

Mixed cryoglobulinemia can cause a false elevation in the platelet count. In this condition, abnormal proteins called cryoglobulins in the blood stick together when exposed to cold temperatures. This can happen in the hands and feet, where circulation is affected by drops in temperature.

These particles may be falsely counted as platelets by the machine performing the complete blood count. This condition is associated with hepatitis C infection, systemic lupus erythematosus, and rheumatoid arthritis.

A rare autoimmune disease, mixed cryoglobulinemia is more common in people between the ages of 40 and 60.

Hemolytic Anemia

SEM image showing glucose in blood
SuperStock / Getty Images

If hemolytic anemia (anemia secondary to red blood cell fragmentation) results in the formation of very small red blood cells, these red blood cells may be inaccurately counted as platelets by the machine that performs the complete blood count.

This type of anemia can be diagnosed by reviewing a peripheral blood smear (a microscope slide of blood). A visual count of the platelets will be lower with many small red blood cells seen.


Platelets with a few red and white blood cells

Science Picture Co / Getty Images

High platelet counts can be a secondary effect of some malignancies (cancers). This is known as paraneoplastic thrombocytosis.

This condition is more common in solid tumors like lung cancer, hepatocellular (liver) carcinoma, ovarian cancer, and colorectal cancer. Elevated platelet counts can also be seen in chronic myelogenous leukemia (CML).

Frequently Asked Questions

  • What are possible complications of thrombocytosis?

    The most dangerous complication for most people with thrombocytosis is blood clots. Pregnant women also may have serious complications that include miscarriage, slowed fetal growth, or placental abruption.

  • Can vitamin D deficiency cause a high platelet count?

    It's possible. Research on this is limited, but a 2020 study found that low vitamin D can cause higher platelet counts. Study subjects had no other diseases or conditions that affected platelets. More research is needed.

  • Can stress cause high platelets?

    Yes. Studies have shown that anxiety, mood disorders, and stress can cause increased platelet counts.

16 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.
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By Amber Yates, MD
Amber Yates, MD, is a board-certified pediatric hematologist and a practicing physician at Baylor College of Medicine.