Essential thrombocythaemia — Rapid

Synopsis

Distinguish essential thrombocythaemia from reactive thrombocytosis and prefibrotic myelofibrosis, assess thrombosis and bleeding risk and use antiplatelet or cytoreductive treatment safely.

ET is a sustained platelet count at least 450 × 10⁹/L with characteristic marrow megakaryocytes, a clonal marker and exclusion of reactive thrombocytosis and other myeloid neoplasms.
First-line assessment is repeat FBC and film, ferritin and iron, CRP or ESR, renal and liver tests and review for infection, inflammation, bleeding, cancer, surgery and splenic status.
Test JAK2 V617F first, then CALR and MPL when negative; test BCR-ABL1 when features are atypical or no clear MPN driver is found.
Bone-marrow trephine is central because ET must be separated from prefibrotic myelofibrosis and masked PV, which cannot be done reliably from platelets alone.
Thrombotic risk uses previous thrombosis, age, JAK2 and cardiovascular risk; the platelet number alone is a poor predictor of thrombosis.
Low-dose aspirin treats microvascular symptoms and prevents arterial events in appropriate risk groups, but withhold it in active bleeding or suspected acquired VWD.
High-risk ET usually receives cytoreduction, commonly hydroxycarbamide plus low-dose aspirin; interferon is preferred when pregnancy or reproductive concerns make hydroxycarbamide unsuitable.
Extreme thrombocytosis can bleed: check VWF activity and reduce the count rather than reflexively prescribing aspirin.

Key red flags

Focal neurological deficit, chest pain, acute limb pain, abdominal splanchnic thrombosis or hypoxia requires immediate thrombosis assessment and treatment.

Mucosal or major bleeding with an extreme platelet count suggests acquired von Willebrand syndrome; antiplatelet therapy may worsen it.

New anaemia, leucoerythroblastosis, progressive splenomegaly, constitutional symptoms or blasts suggests myelofibrotic or leukaemic transformation.

Major thrombosis

Stroke, coronary, DVT, PE and portal, hepatic or cerebral venous events can occur even with a modest platelet count.

Acquired VWD bleeding

Epistaxis, gum or gastrointestinal bleeding with extreme thrombocytosis suggests loss of functional high-molecular-weight VWF.

Investigation priorities

First-line reactive-cause screenFirst stepFirst line

Confirm persistence and identify common non-clonal platelet drivers.

Management branches

Persistent platelets above 450Exclude reaction and classify marrow

Thrombocytosis persists on repeat FBC without an obvious transient cause.

Assess film, iron, inflammation, infection, bleeding, malignancy, surgery and spleen.
Test JAK2, then CALR and MPL, adding BCR-ABL1 when appropriate.
Obtain expert marrow trephine and integrate morphology and genetics before assigning ET and risk.

Key medicines

Low-dose aspirinGive 75 mg orally once daily when the thrombotic or microvascular indication outweighs bleeding risk; do not start during active bleeding or suspected acquired von Willebrand syndrome until assessed.Review gastrointestinal bleeding, anticoagulation, pregnancy and VWF function with extreme platelets. Twice-daily dosing is specialist only and not a routine response to a high count.

HydroxycarbamideStart commonly at 500 mg orally once or twice daily and titrate with frequent FBC to safe platelet and leukocyte control, adjusting the starting and maintenance dose for renal function, age and cytopenia.Monitor cytopenia, renal function, mouth or leg ulcers and skin cancer, avoid pregnancy and breastfeeding and use effective contraception; discuss fertility and interferon when reproductive plans matter.

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