Essential thrombocythaemia
A myeloproliferative neoplasm in which clonal megakaryocyte proliferation, usually driven by JAK2, CALR or MPL, sustains a high platelet count; the raised, functionally abnormal platelets cause both thrombosis and, paradoxically at very high counts, bleeding, so it is a diagnosis of exclusion once reactive thrombocytosis is ruled out.
Definition
Essential thrombocythaemia is a myeloproliferative neoplasm defined by a sustained increase in the platelet count due to clonal proliferation of megakaryocytes, in the absence of a reactive cause or another myeloid neoplasm. Most cases carry a JAK2, CALR or MPL driver mutation.
First principles
Clonal megakaryocytes overproduce platelets
A driver mutation activates JAK-STAT signalling in the stem cell and skews it toward the megakaryocyte lineage, so platelets are made autonomously and the count stays persistently high. Because this is a clonal, unregulated process rather than a response to a stimulus, the thrombocytosis is sustained and is accompanied in most cases by a demonstrable driver mutation, unlike reactive thrombocytosis.
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