Thrombocytosis

Thrombocytosis, also called thrombocythemia, is the term used when the platelet count goes above 450,000 per microliter of blood. Platelets are the blood component responsible for clotting, and they arise in the bone marrow from large cells called megakaryocytes.

That high number can arise by two different routes, and telling them apart matters. Primary thrombocytosis arises from unregulated production in the marrow progenitors themselves, and it is usually associated with myeloproliferative neoplasms, a group of diseases in which the marrow overproduces blood cells. The secondary form, also called reactive, is a high count in response to something else: an underlying illness, an infection or certain medications. These are two ways to fill the same warehouse: production control broke down, or an order came in from outside.

The reactive form, which is the more common one, has a recognizable mechanism. Inflammation drives up interleukin 6, a chemical signal of the immune system, and that in turn stimulates megakaryocytes to make platelets and prompts the liver to produce more thrombopoietin, the hormone that directs that production.