Secondary Thrombocytosis

Once referred to ignobly as "blood dust," platelets are a component of blood produced in the bone marrow that have a vital role in the blood clotting process. The average platelet count in adults and children is usually between 150,000 and 450,000/μL (150 to 450 x 10/L), although the normal range may vary in different clinical laboratories. Thrombocytosis, or thrombocythemia, occurs when the platelet count exceeds 450,000/μL of blood.

Thrombocytosis can be classified into primary and secondary (or reactive) thrombocytosis. This distinction is essential as it carries implications for evaluation, prognosis, and treatment. Primary thrombocytosis results from an unregulated abnormality in platelet production by bone marrow progenitor cells and is usually associated with myeloproliferative neoplasms. Primary thrombocytosis, especially in conditions such as essential thrombocythemia and polycythemia vera, carries an increased risk of thrombosis and bleeding compared to secondary thrombocytosis.

Secondary thrombocytosis, also known as reactive thrombocytosis, is characterized by an abnormally high platelet count due to underlying events, infections or diseases, or certain medications. Secondary thrombocytosis, which is more common than primary thrombocytosis, is typically identified through routine laboratory studies. In most cases, secondary thrombocytosis symptoms are due to an underlying disorder rather than the thrombocytosis itself. Rarely, extreme thrombocytosis may lead to thrombotic events such as acute myocardial infarction, mesenteric vein thrombosis, and pulmonary embolism.

Although secondary thrombocytosis is typically benign, the underlying causes—such as malignancy, connective tissue disorders, and chronic infections—can be associated with an increased risk of adverse outcomes. Approximately 80% to 90% of individuals with thrombocytosis are known to have secondary thrombocytosis. Causes of secondary thrombocytosis include transient conditions like acute blood loss or infection, as well as sustained factors such as iron deficiency, asplenia, cancer, chronic inflammation, or infectious diseases. Reactive thrombocytosis is a laboratory anomaly that generally resolves once the underlying causative condition is addressed.