Thrombocytosis is an abnormally high platelet count, usually > 400 * 10^9/l.

Reactive (Secondary)

• Infection, inflammation, trauma
• Iron deficiency
• Malignancy
• Post‑splenectomy (hyposplenism)

Clonal (Primary)

• Essential thrombocythaemia
• Polycythaemia vera
• Primary myelofibrosis
• CML

Clues for clonal: persistent high platelets, splenomegaly, abnormal megakaryocytes, JAK2/CALR/MPL mutation.

Essential thrombocytosis

Essential thrombocytosis is one of the myeloproliferative disorders which overlaps with chronic myeloid leukaemia, polycythaemia rubra vera and myelofibrosis.

Essential thrombocythemia is a stem cell disorder marked by a predominant increase in megakaryocytes and platelet count >600,000, in the absence of secondary thrombocytosis

Features

• platelet count > 600
• both thrombosis (venous or arterial) and haemorrhage can be seen
• a characteristic symptom is a burning sensation in the hands
• a JAK2 mutation is found in around 50% of patients