What Causes a High Platelet Count?

Infection

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

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

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

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.

Inflammatory Conditions

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

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

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.

Cancer

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).