Investigating thrombocytosis

### What you need to know 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. 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 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 ### DefinitionsRETURN TO TEXT