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Haematology & Oncology

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.

In a nutshell

ET is a clonal megakaryocytic neoplasm causing persistent thrombocytosis. Thrombosis and bleeding risk—not the platelet count alone—drive aspirin and cytoreductive decisions.

Classic presentation

An incidental persistent thrombocytosis, or microvascular symptoms, thrombosis or bleeding without a clear reactive cause.

Key points

  • Exclude reactive thrombocytosis and iron deficiency.
  • JAK2, CALR and MPL testing plus marrow morphology support diagnosis.
  • Extreme thrombocytosis can cause acquired von Willebrand bleeding.
  • Aspirin is risk- and bleeding-adapted, not automatic.
  • Cytoreduction is reserved for selected higher-risk or symptomatic disease.

First-line investigation

Repeat FBC and film, exclude reactive causes, then use mutation testing and specialist marrow assessment.

Management

Confirm persistent clonal thrombocytosis

  • Exclude reactive causes and use mutation testing, blood film and marrow assessment through specialist haematology; mutation-negative cases need careful differential diagnosis.1,2

Reduce vascular risk

  • Assess previous thrombosis, age, cardiovascular risks, symptoms and bleeding before selecting antiplatelet treatment.1,3,5

Cytoreduce selected patients

  • Use cytoreduction for high-risk disease or clinically important symptoms, with pregnancy, age, comorbidity and acquired von Willebrand risk influencing choice.1,2,5

Monitor thrombosis, bleeding and progression

  • Follow counts, symptoms, treatment toxicity, acquired von Willebrand syndrome and progression to post-ET MF or AML.1,2

Exam traps

  • High platelets do not equal ET.
  • ET can cause both thrombosis and bleeding.
  • Aspirin can be harmful with acquired von Willebrand syndrome.
  • Mutation-negative thrombocytosis needs a broad differential.

Illustrations

Bone marrow smear in essential thrombocythaemia showing markedly increased platelets, including large formsA blood film showing markedly increased platelets, including large forms, with otherwise unremarkable red and white cells, typical of essential thrombocythaemia.No machine-readable author provided. KGH assumed (based on copyright c, Wikimedia Commons · CC-BY-SA-3.0
ErythromelalgiaA clinical photograph of erythromelalgia showing red, warm, painful extremities from platelet-mediated microvascular occlusion.Herbert L. Fred, MD and Hendrik A. van Dijk, Wikimedia Commons · CC-BY-2.0

Key sources

  1. BSH: Guideline for investigation and management of adults and children presenting with thrombocytosis (UK BSH diagnostic and management guidance relevant to thrombocytosis and ET.)Published 1 Nov 2010
  2. BSH: Investigation and management of thrombocytosis without JAK2, CALR or MPL mutations (Current UK BSH guidance for mutation-negative thrombocytosis and differential diagnosis.)Published 4 Dec 2025
  3. BSH: Diagnosis and management of polycythaemia vera (UK BSH diagnostic, risk-stratification and management guidance for PV.)Published 27 Nov 2018
  4. NHS Salisbury: Myeloproliferative neoplasms (UK NHS overview of MPNs and specialist pathways.)
  5. BNF online (Check current antiplatelet, cytoreductive, JAK-inhibitor and supportive prescribing.)

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.