Essential thrombocythemia

Synonyms: primary thrombocythemia, essential thrombocytosis, too many platelets, myeloproliferative neoplasm
Specialty: Internal medicine · Haematology & oncology
Images: Blood smear & cytology 1 · Clinical 1
Last updated: 10/2026 · Dr. Pascal Bafteh

Contents

Images (2)
Definition
Occurrence & epidemiology
Aetiopathogenesis
Clinical features
Diagnosis
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Further reading (open access)
Cross-references

Images (2)

Essential thrombocythemia – Blood smear in essential thrombocythemia: masses of platelets (about 1.5–2 million/µl), some large, between the red cells — Blood smear in essential thrombocythemia: masses of platelets (about 1.5–2 million/µl), some large, between the red cellsImage: Ed Uthman from Houston, TX, USA (Wikimedia Commons) · CC BY 2.0 · Source

Essential thrombocythemia – clinical photo: Erythromelalgia: intensely reddened hand and fingers as a microvascular symptom of a myeloproliferative neoplasm — Erythromelalgia: intensely reddened hand and fingers as a microvascular symptom of a myeloproliferative neoplasmImage: Minor (Wikimedia Commons) · CC BY-SA 4.0 · Source

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Definition

Essential thrombocythemia (ET) is a myeloproliferative neoplasm with a persistently increased platelet count, megakaryocytic hyperplasia in the bone marrow, and a tendency to bleeding and microvascular circulatory disturbances. It is a diagnosis of exclusion after reactive thrombocytosis and other myeloproliferative neoplasms have been ruled out.

Occurrence & epidemiology

ET usually occurs after age 50; women are affected more often.

Aetiopathogenesis

It is a clonal disorder of the hematopoietic stem cell with increased platelet production. Driver mutations:

JAK2 V617F in about 50 %
CALR mutations (exon 9, type 1 and type 2), usually with higher platelet counts
MPL mutations (thrombopoietin receptor) in a few patients
Rarely, all three are negative (triple-negative).

With extreme thrombocytosis, platelets adsorb and cleave large von Willebrand multimers, causing type 2 acquired von Willebrand syndrome.

Clinical features

Microvascular disturbances: erythromelalgia (burning pain, warmth, and redness of the hands and feet, sometimes with digital ischemia and ulcers), headache and ocular migraine, paresthesias, transient ischemic attacks
Thrombosis, arterial or venous
Bleeding: usually mild and rarely spontaneous, e.g., epistaxis, bruising, or gastrointestinal bleeding; serious bleeding mainly with extreme thrombocytosis around 1,000,000/µL
The spleen may be palpable; marked splenomegaly suggests another myeloproliferative neoplasm.

Diagnosis

Platelet count above 450,000/µL, sometimes above 1,000,000/µL; hematocrit, white cell count, MCV, and iron studies normal.
Peripheral smear: giant platelets and megakaryocyte fragments.
Iron studies, because iron deficiency can cause reactive thrombocytosis.
Molecular genetics: quantitative JAK2 V617F assay and BCR::ABL1 analysis (exclusion of CML); if negative, CALR and MPL. The JAK2 allele burden in ET does not exceed 50 %; higher values suggest PV or PMF.
Bone marrow biopsy in selected cases: increased numbers of enlarged, mature megakaryocytes.
Reactive thrombocytosis, CML, masked PV, and myelodysplastic neoplasms with thrombocytosis (e.g., 5q syndrome) are to be distinguished. In about 25 %, a disease initially appearing as ET evolves into overt PV over time.

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