Synopsis
Distinguish reactive from clonal thrombocytosis, recognise paradoxical bleeding and acquired platelet dysfunction, and direct urgent or definitive treatment at the mechanism rather than the platelet count alone.
- Confirm genuine thrombocytosis with a repeat FBC and film; the platelet count alone does not distinguish a reactive process from a myeloproliferative neoplasm.
- First-line cause assessment includes history, examination, serial counts, ferritin and iron studies, CRP or ESR, renal and liver function and review for infection, inflammation, bleeding, surgery, splenectomy and malignancy.
- Persistent unexplained thrombocytosis requires haematology assessment with JAK2 V617F, then CALR and MPL or an appropriate myeloid panel; BCR::ABL1 testing and marrow examination depend on phenotype.
- A high count does not automatically require aspirin or cytoreduction: reactive thrombocytosis is treated by correcting the driver, while confirmed clonal disease is managed by age, thrombosis history, mutation and bleeding risk.
- Extreme thrombocytosis with bleeding is a warning for acquired von Willebrand syndrome; check VWF activity and antigen and withhold aspirin until haemostasis risk is understood.
- Acquired platelet dysfunction can cause bleeding with a normal count, PT and APTT. Diagnose from phenotype, medicine exposure, renal or systemic disease and specialist functional testing when it will change care.
- For serious uraemic bleeding or an urgent procedure, correct anaemia and dialysis factors and consider desmopressin 0.3 micrograms/kg intravenously over 20–30 minutes under specialist supervision.
- Platelet transfusion is reserved for clinically important bleeding or urgent procedures where expected benefit exceeds thrombosis risk; recent antiplatelet exposure alone is not a universal indication.
Key red flags
New focal neurology, chest pain, painful pale limb, abdominal pain or unusual-site thrombosis may be a clonal myeloproliferative presentation even when thrombocytosis was previously labelled reactive.
Recurrent epistaxis, gastrointestinal bleeding or postoperative oozing with platelets above approximately 1,000 × 10⁹/L suggests acquired von Willebrand syndrome and makes empirical aspirin hazardous.
Uraemia, severe liver disease, cardiopulmonary bypass or recent potent antiplatelet therapy can cause major platelet dysfunction despite a normal platelet count and routine coagulation tests.
Mucosal bleeding at an extreme count is not reassurance against clotting; it requires acquired VWF assessment before aspirin.
Investigation priorities
Verify persistence and identify spurious counts or another myeloid phenotype.
Find common reversible drivers before labelling a clonal disorder.
Management branches
An automated platelet count is at least 450 × 10⁹/L or unexpectedly rising.
- Repeat the FBC and inspect a film, comparing prior counts and the timing of acute illness or surgery.
- Assess iron status, inflammation, infection, bleeding, malignancy, splenectomy and renal or liver disease from history, examination and targeted tests.
- Treat a demonstrated secondary cause and document a repeat count; refer persistent unexplained or phenotypically concerning cases to haematology.