Intended for healthcare professionals

Practice Rational Testing

Investigating thrombocytosis

BMJ 2019; 366 doi: (Published 04 July 2019) Cite this as: BMJ 2019;366:l4183

This article has a correction. Please see:

  1. Abhinav Mathur, academic foundation year 2 doctor1,
  2. Shehan Samaranayake, general practitioner2,
  3. Neill PF Storrar, specialist haematology registrar3,
  4. Mark A Vickers, professor of haematology1
  1. 1School of Medicine & Dentistry, University of Aberdeen, Scotland, UK
  2. 2Howden Health Centre, Livingston, Scotland, UK
  3. 3Department of Haematology, Western General Hospital, Edinburgh, Scotland, UK
  1. Correspondence to A Mathur abhinav.mathur{at}

What you need to know

  • Thrombocytosis is usually reactive or caused by clonal disorders

  • Initial assessment includes repeat history and examination, a peripheral blood smear examination, and determination of iron and acute phase reactant status

  • If no cause of inflammation is found, consider investigations for an occult malignancy or seek specialist advice for investigation of a clonal haematopoietic disorder

A 47 year old woman presents to her general practitioner complaining of feeling tired all the time. She is an ex-smoker and has a history of irritable bowel syndrome. Examination is normal. A full blood count is normal except for a platelet count of 725×109/L.

How common is thrombocytosis?

Thrombocytosis is defined as a platelet count elevated more than two standard deviations above the population mean, typically >400-450×109/L, and therefore includes 2.3% of the population.1 Reference ranges usually do not account for variation in platelet counts dependent on age, sex, and ethnicity,23 such that the upper limits of normal should be lower in older individuals and men.4 Approximately 25% of the UK adult population attending primary care will have a full blood count (FBC) in any one year.5 Thrombocytosis is a common incidental finding in 1.5% to 2.2% of the population aged >40 consulting primary care.6

What causes thrombocytosis?

The differential diagnosis for thrombocytosis is broad (table) and the diagnostic process can be challenging.7 Rarely, non-platelet structures in peripheral blood can be erroneously counted as “platelets” in automated FBC counters, leading to a spurious thrombocytosis.8 The two main classes of genuine thrombocytosis are secondary or reactive causes and primary or clonal causes (ie, haematological neoplasms) (box 1). In one cohort study of 732 people with an elevated platelet count, the thrombocytosis in 80-90% of patients was reactive to an underlying inflammatory cause.9

View this table:

Causes of thrombocytosis

Box 1


  • Reactive thrombocytosis—proliferation of platelets …

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