Thrombocytosis (High Platelets)


Thrombocytosis is an increase in platelets more than 500 x 10 9/L, commonly found incidentally in a routine blood test.

There are broadly two types of thrombocytosis:

Primary haematological disease:

  • Essential thrombocythaemia (also referred to as primary thrombocytosis). The clinical features can relate to an increased incidence of arterial and venous thrombosis and bleeding. Most can remain well for decades.
  • Polycythaemia vera
  • Primary myelofibrosis
  • Chronic myeloid leukaemia
  • Myelodysplastic syndromes

Secondary or reactive (changes are caused by an exaggerated physiological response to a primary problem):

  • infectious disease
  • inflammatory disease
  • neoplasms
  • non-malignant haematological conditions e.g. acute blood loss, iron deficiency anaemia
  • functional and surgical hyposplenism
  • tissue damage, e.g. recent trauma or surgery
  • exercise
  • reaction to medications e.g. steroids, adrenalin
  • pregnancy
  • allergic reactions


Signs and Symptoms

  • History of arterial or venous thromboembolism or bleeding
  • Look for inflammatory, infective, or neoplastic conditions
  • Examine for splenomegaly

History and Examination

  • History of arterial or venous thromboembolism or bleeding


  • Full Blood Count
  • Ferritin
  • C-reactive Protein (CRP)


  1. If haematocrit (Hct) is elevated or splenomegaly, consider primary polycythaemia vera and request haematology assessment
  2. If haematocrit is normal and no splenomegaly, repeat full blood count, ferritin, and CRP in 2 to 3 weeks
  • If ferritin is low, consider iron deficiency.
  • Thrombocytosis is most likely to be reactive if:
    • platelets normalise with time
    • platelets and CRP remain elevated (ferritin may be raised)
    • if underlying reactive condition present (see list above)
  • If platelets remain more than 500 x 109/L but CRP and ferritin are normal, it is likely to be essential thrombocythaemia and request haematology assessment.


Referral Criteria

  • Request haematologist assessment if platelets remain more than 500 x 109/L.
  • Where appropriate, written advice may also be available

Referral Instructions

e-Referral Service Selection

  • Specialty: Haematology
  • Clinic Type: Not otherwise specified
  • Service: DRSS-Northern-Haematology- Devon CCG- 15N

Referral Forms

DRSS Referral Form

Supporting Information

Pathway Group

This guideline has been signed off by NEW Devon CCG.

Publication date: July 2016


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