Thrombocytosis (Guidelines)

Thrombocytosis is a common finding and is a frequent cause of referral for further investigation. There is a wide range of primary (eg, essential thrombocythaemia (ET), other myeloproliferative neoplasm MPN) and secondary causes as well as false or ‘spurious’ conditions mimicking thrombocytosis. Much less commonly there is a primary cause (eg, ET, other MPN). Establishing the cause therefore requires consideration of clinical features, haematological parameters and other test results.

The most common secondary (or reactive) causes of thrombocytosis are infection, inflammation, iron deficiency, tissue damage, haemolysis, severe exercise, malignancy, hyposplenism and other causes of an acute phase response. These are usually, but not always, characterized by an elevated C-reactive protein.

Identified on FBC, either incidentally or in the context of other clinical features.

• Check ferritin, CRP, blood film.
• Exclude other causes of inflammation, including infection, malignancy, surgery.
• History to include above, and specifically if there has been previous splenectomy.
• Further imaging may be helpful to inform the assessment.
• JAK2/CALR/MPL/BCR::ABL molecular mutation testing if no other obvious explanation. 
  If positive this is specific for primary causes of thrombocytosis (although a negative result does not exclude it), but the majority of cases are secondary.
• Further investigation: this will be directed by secondary care after referral or consultation.

Note re ICE test availability (Nov 2023):

There has been a delay in the next set of tests to be included in the next addition for ICE. Any test not already on ICE can be requested using the 'Test not listed on ICE' functionality (users should have received training by the eHealth facilitators). NB: a paper request can be sent for infrequent or unusual requests.

• JAK2/CALR/MPL/BCR::ABL mutation positive cases.
• Mutation negative cases in the absence of another precipitant.

• As per primary care guidance.
• Note that molecular testing may be best deferred to community testing in order to exclude reactive thrombocytosis in the context of illness.

Associated thrombosis or bleeding.