What Does High Platelet Count Mean?

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What are some of the potential diagnoses when a person has a high platelet count (platelet count approximately 420)?

Doctor's response

Platelets are the smallest cell-like structures in the blood and are important for blood clotting and plugging damaged blood vessels. Platelet counts are usually done by laboratory machines that also count other blood elements such as the white and red cells. Normal platelet counts are in the range of 150,000 to 400,000 per microliter (or 150 - 400 x 109 per liter), but the normal range for the platelet count varies slightly among different laboratories.

An elevated platelet count is known as thrombocytosis. The laboratory where the test was performed will be able to tell whether a platelet count of 420,000 represents an elevated value for this laboratory. An elevated platelet count can be observed in people without significant medical problems. Mild to moderately elevated platelet counts are commonly seen when chronic inflammation is present. In other cases a high platelet count can signal a more serious blood problem known as a myeloproliferative disorder (abnormal growth of blood cell elements within the bone marrow). Your doctor may need to perform additional tests to determine the cause of an abnormally high platelet count.

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Last Editorial Review: 6/19/2017
Medically reviewed by John A. Daller, MD; American Board of Surgery with subspecialty certification in surgical critical care


"Approach to the patient with thrombocytosis"